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TC 
824 
C2 
A2 
no.  113 


LIBRARY 

UNIVERSITY  OF  CALIFORNIA 


LIBRARY 
UNIVERSITY  OF  CALIFORNIA 


THE   RESOURCES   AGENCY   OF    CALIFORNIA 
partment    of    Water    Resources 


BULLETIN  No.  113 

VEGETATIVE 

WATER  USE  STUDIES 

1954-1960 

Interim  Report 


UNIVERSITY    or 


LIdRARY 


AUGUST  1963 


HUGO  FISHER 

Adminisfrafor 
The  Resources  Agency  of  California 


EDMUND  G.  BROWN 

Governor 
State  of  California 


WILLIAM  E.  WARNE 

Director 

Department  of  Water  Resources 


stale    ol    California 
THE    RESOURCES    AGENCY    Of    CALIEORNIA 

Department    of    Water    Resources 


BULLETIN  No.  113 

VEGETATIVE 

WATER  USE  STUDIES 

1954-1960 


Interim  Report 


AUGUST   1963 


HUGO  FISHER  EDMUND  G.   BROWN  WILLIAM  E.  WARNE 

Adminisirafor  Governor  Director 

The  Resources  Agency  of  California  State  of  California  Department  of  Water  Resources 


LIBRARY 

UNlVKR.<;iTV  OR  PAT  TTrnPlMTX 


ERRATA 

Bulletin  No.  113,  '•Vep:etatlve  V/ater  Use  Studies,  195^  -  196O" 
Plate  1,  Agrocllmatlc  Stations  No.  h,    57.  61,  75,  93,  101: 

For  "Inactive  -  i960"  read  "Active  -  196C" 
Plate  2,    For  "Evapotransperometer"  read  "Evapotranspirometer" 
Page  58,  line  10,  For  "Figures  E  and  F"  read  "Figures  A  and  B' 
Page  66,  line  21 ,  For  "Figures  A  and  B"  read  "Figures  E  and  F' 


TABLE  OF  CONTENTS 

Page 

lET'lER  OF  TRANSMITTAL vii 

ORGANIZATION,  DEPARTMEJIT  OF  WATER  RESOURCES viii 

ORGANIZATION,  CALIFORNIA  WATER  COMMISSION ix 

ACKNOWLEDGMENT   x 

CHAPTER  I.  INTRODUCTION 1 

Need  for  Vegetative  Water  Use  Studies 1 

Authorization  2 

Objective •  3 

Scope  of  Present  Program  and  Report •  h 

CHAPTER  II.  AGROCLIMATIC  MONITORING  PROGRAM 7 

Instrumentation  at  Agroclimatic  Stations  •       8 

Atmometers  8 

Evaporation  Pans 9 

Agroclimatic  Data  Analysis  9 

CHAPTER  III.  EVAPOTRANSPIRATION  MEASUREMENT   21 

Measurement  of  Data  Related  to  Evapotranspiration  22 

Criteria  for  Selection  of  Plots 2i| 

Evapotranspiration  Measurement  Technique  and  Discussion  of 

Development  and  Current  Methods   25 

Field  Plot  Sampling  Neutron  Scattering  Technique  29 

Pittville  Neutron  Probe  Moisture  Depletion 

Measurements 31 

Arvin  Neutron  Probe  Moisture  Depletion  Measurements  .  ,  3li 


iii 


TABLE  OF  CONTENTS  (continued) 

Page 

Evapotranspirometer  Measiirements 36 

Alturas-Dorris  Ranch  Evapotranspirometer  Measxirments  ....  37 

Coleville  Evapotranspirometer  MeasTirements   .........  39 

Davis  Evapotranspirometer  Keasurments   UO 

Evapotranspiration  Data  Summary iiO 

CHAPTER  IV.  CORRELATION  OF  EVAPOTRANSPIRATION  DATA 

WITH  AGROCLIMATIC  DATA 51 

Evapotranspiration  and  Climatic  Data  ............  52 

Evapotranspiration  and  Plant  Conditions  53 

Evapotranspiration  and  Soil  Moisture   ....•  5U 

Other  Factors  Affecting  Evapotranspiration  55 

Determination  of  Coefficients  55 

Grass  and  Pasture  Coefficients 57 

Alfalfa  Coefficients 58 

Cotton  Coefficients   63 

Application  of  Coefficients  and  Evaporation  Data  to 

Estimation  of  Evapotranspiration  , 67 

CHAPTER  V.   SUMMARY,  CONCLUSIONS  MD  RECOMMENDATIONS  .  .  71 

Suininary 71 

Conclusions • 7h 

Recommendations  ....  .... .....•.••  75 

Appendix  A.  Supplemental  Agroclimatic  and  Evapo- 
transpiration Data 77 


iv 


TABIE  OF  CONTENTS  (continued) 


TABLES 


Nimber 


1  Mean  Monthly  Evaporation  From  Standard 

U.  S.  Weather  Bureau  Evaporation  Pans 12 

2  Mean  Monthly  Evaporation  Difference  Between 

Livingston  Spherical  Black  and  White  Atmometers.  .      12 

3  Monthly  Evaporation  From  Standard  U,  S.  Weather 

Bureau  Evaporation  Pans  in  Order  of  Decreasing 

Magnitude  For  Irrigated  Pasture  and  Dryland 

Stations 16 

k             Monthly  Evaporation  Difference  Between  Livingston 
Spherical  Black  and  VJhite  Atmometers  in  Order  of 
Decreasing  Magnitude  For  Irrigated  Pasture  and 
Dryland  Stations  19 

5  Summary  of  Measurements  of  Evapotranspiration  and 

Related  Data U2 

6  Pan  and  Atmometer  Coefficients  for  Pasture  and  Grass      59 

7  Pan  and  Atmometer  Coefficients  for  Alfalfa  6U 

8  Pan  and  Atmometer  Coefficients  for  Cotton  68 

9  Comparison  of  Seasonal  Consumptive  Use  of  Alfalfa, 

Pasture,  and  Cotton  Based  on  Bulletin  No.  2 

Growing  Season  69 

FIGURES 

Page 

Atmometer  Assembly   .,  10 

Typical  Agroclimatic  Stations   11 

Platforms  Used  to  Minimize  Crop  Damage  and  Soil 

Compaction 32 

h  Access  Tube  Design 33 

5     Evapotranspirometer  Designs   «      Ul 


Number 
1 
2 
3 


TABLE  OF  CONTENTS  (continued) 


PLATES 
(Bo\ind  at  End  of  Bulletin) 

Number 

1  General  Location  of  Agroclimatic  Stations 

2  General  Location  of  Evapotranspiration  Stations 

3  Comparison  of  Evapotranspiration  Curves  of  Different 

Crops  Grown  at  the  Same  Location  on  the  Same  Soil 
Series 

h  Variation  of  Pan  and  Atmometer  Coefficients  for 

Individual  Periods  of  Measurements 

5  Comparison  of  Pan  and  Atmometer  Coefficients  for 

Cotton,  Alfalfa  and  Grass 

6  Jlelationship  Between  Pan  and  Atmometer  Coefficients 

For  Alfalfa  and  Ground  Cover 


EDMUND  G.  BROWN 
GOVERNOR  OF 
CALIFORNIA 


HUGO  FISHER 

ADMINISTRATOR 

RESOURCES  AGENCY 


THE    RESOURCES    AGENCY    OF    CALIFORNIA 

DEPARTMENT  OF  WATER  RESOURCES 

1120  N.  STREET,  SACRAMENTO 

June  20,  1963 


Honorable  Edmund  G.  Brovm,  Governor 
and  Members  of  the  Legislature 
of  the  State  of  California 

Gentlemen: 

I  have  the  honor  to  transmit  herewith  Bulletin 
No.  113,  "Interim  Report  on  Vegetative  Water  Use  Studies, 
19514-1960,"  of  the  Department  of  V/ater  Resources,  dated 
May  1963.  This  report  describes  techniques  and  approaches 
■which  have  evolved,  and  summarizes  data  on  vegetative  con- 
sumptive use  or  evapotranspiration.   Interrelationships 
between  these  data  are  set  forth,  together  with  evapotran- 
spiration values  for  some  crops  in  Central  and  Northern 
California  agricultural  areas.  This  is  a  continuing  study 
with  many  conclusions  yet  to  be  reached. 

Data  pertaining  to  evapotranspiration,  irrigation 
requirements,  and  agricultural  hydrology  are  basic  to  most 
water  resource  development  studies.  With  the  continued 
growth  of  the  State,  necessitating  more  complex  and  costly 
water  development  facilities,  there  is  increasing  need  for 
more  accurate  water  use  data.  Such  data  will  enable  de- 
veloped surface  and  ground  water  resources  to  be  used 
effectively,  and  will  facilitate  design  and  operation  of 
land  drainage  systems. 

The  studies  reported  herein  were  initiated  in  195U 
as  part  of  the  Northeastern  Counties  Investigation.  A  con- 
tinuing Vegetative  Water  Use  Studies  Program  was  established, 
and  the  studies  were  broadened,  sls  a  result  of  Senate  Bill  Ii3li; 
1959  Legislative  Session.  Specific  authorization  for  these 
studies  is  set  forth  in  Section  226(e)  of  the  Water  Code. 


Sincerely  yours ^ 


'  '  T)t  rpct.OT 


STATE  OF  CALIFORNIA 

THE  RESOURCES  AGENCY  OF  CALIFORNIA 

DEPARTMENT  OF  WATER  RESOURCES 


EDMUND  G.  BROWN,  Governor 

HUGO  FISHER,  Administrator,  The  Resources  Agency  of  California 

VJILLIAM  E.  WARNE,  Director,  Department  of  VJater  Resources 

ALFRED  R.  GOLZE,  Chief  Engineer 


Division  of  Reso\irces  Planning 

William  L.  Berry,  Division  Engineer 
Albert  J.  Dolcini,  Chief,  Planning  Management  Branch 

Technical  studies  were  conducted  and 
the  bulletin  was  prepared  under  the  supervision  of 


John  W.  Shannon 

Reginald  E.  Merrill 
Norman  MacGillivray 


Water  Utilization  Staff  Specialist 
Assisted  by 

Associate  Land  and  Water  Use  Analyst 
Assistant  Land  and  Water  Use  Analyst 


Field  data  were  collected  under 
the  supervision  of 

Jack  H.  Lawrence  Senior  Land  and  Water  Use  Analyst 

Assisted  by 

John  Kono  Assistant  Land  and  Water  Use  Analyst 

Andrew  Lee  Junior  Land  and  Water  Use  Analyst 

Arthur  deRutte  Assistant  Land  and  Water  Use  Analyst 

Patrick  Duval  Assistant  Land  and  Water  Use  Analyst 

Robert  Bowman  Assistant  Land  and  Water  Use  Analyst 

Victor  Uhlik  Assistant  Land  and  Water  Use  Analyst 

Darrell  Nichols  Assistant  Land  and  Water  Use  Analyst 

Zene  Bohrer  Assistant  Land  and  Water  Use  Analyst 


viii 


CALIFORNIA  WATER  CCMMISSION 

RALPH  M.  BRODY,  Chairman,  Fresno 
WILLIAM  H.  JF^INGS,  Vice  Chairman,  La  Mesa 

JOHN  W.  BRYANT,  Riverside  JOHN  P.  BUNKER,  Gustine 

IRA  J.  CHRISMAN,  Visalia  JOHN  J.  KING,  Petaluma 

EDWIN  KOSTER,  Grass  Valley  NORRIS  POULSON,  La  Jolla 

MARION  R.  WALKER,  Ventura 


0- 


WILLIAM  M.  CARAH 
Executive  Secretary 


GEORGE  B.  GLEASON 
Principal  Engineer 


ACKNOVJLEDGElffiOT 

The  Department  of  VJater  Resources  wishes  to  express 
appreciation  to  many  organizations  and  individuals  who  have 
assisted  the  department  in  the  Vegetative  Water  Use  Program. 
I-lany  private  farm  operators  have  provided  use  of  their  property 
and  equipment,  as  well  as  time.  The  list  is  too  numerous  to 
completely  enumerate;  however,  the  Frick  Farms  at  Arvin,  Roland 
Hutchings  at  Pittville,  and  the  U.  S.  Fish  and  Wildlife  Service 
(formerly  Dorris  Ranch)  at  Alturas  have  been  particularly 
helpful. 

A  very  considerable  amount  of  technical  guidance  has 
been  given  by  the  Irrigation  Department  of  the  University  of 
California  at  Davis.  The  University  Agricultural  Extension  has 
given  assistajice  in  the  search  for  plot  sites. 

The  assistance  and  collaboration  provided  by  the 
U.  S.  Forest  Service,  the  Agricultural  Research  Service  and  the 
Soil  Conservation  Service  of  the  U.  S.  Department  of  Agriculturej 
the  California  Division  of  Forestry;  and  the  Agricultural 
Commissioner's  Office,  to  mention  a  few,  are  likewise  gratefully 
acknowledged. 


CHAPTER  I,   INTRODUCTION 

This  report  presents  data  on  vegetative  consumptive  use 
of  water,  or  evapotransplratlon,  together  with  certain  Interrela- 
tionships with  agricultural  climatic  factors  Influencing  such  use. 
The  findings  summarized  cover  the  period  195^-1960,  and  represent 
a  large  quantity  of  Individual  measurements  of  evapotransplratlon 
and  related  agricultural  climatic  data.   The  measurements  of  evapo- 
transplratlon represent  scores  of  soil  samples,  neutron  probe  read- 
ings, and  evapotranspirK)meter  measurements  of  Irrigated  alfalfa, 
pasture,  plums,  cotton,  and  grass  crops.   Agricultural  climatic  or 
agrocllmatlc  data  are  likewise  summarized  from  a  large  number  of 
measurements  of  evaporation  from  pans  and  atmometers.   Certain  other 
agrocllmatlc  data,  such  as  measurements  of  solar  radiation  and  rela- 
tive humidity,  were  collected  at  a  few  stations.   These  data  have 
not  been  analyzed  as  yet,  and  will  be  reported  in  later  publications, 

Need  for  Vegetative  Water  Use  Studies 
Historically,  irrigated  agriculture  has  been  the  largest 
user  of  our  developed  water  resources.   This  condition  probably 
will  continue  indefinitely.  The  Department  of  Water  Resources, 
hereinafter  referred  to  as  the  department,  and  its  predecessor 
agencies,  have  made  many  measurements  of  water  deliveries  for  agri- 
cultural uses  with  regard  to  water  right  adjudication.   However, 
for  broad  planning  purposes  the  department  has  relied  largely  upon 


empirical  methods  for  estimating  seasonal  values  of  evapotransplra- 
tion  or  consumptive  use  for  various  crops.   State  Water  Resources 
Board  Bulletin  No,  2,    "Water  Utilization  and  Requirements  of  Cali- 
fornia, 1955/'  has  been  the  primary  source  for  such  estimates. 

As  more  complex  and  costly  water  development  facilities 
are  contemplated,  more  accurate  values  for  irrigation  requirements 
and  evapotranspiration  v/ill  be  needed.   The  location  and  sizing  of 
reservoirs,  distribution  systems,  and  final  disposal  or  drainage 
systems  are  dependent  upon  accurate  estimates  of  at  least  monthly 
values  of  irrigation  requirements  and  evapotranspiration  for  various 
kinds  of  vegetation.  Accurate  irrigation  requirements  and  evapo- 
transpiration values  are  also  important  in  planning  for  the  con- 
junctive operation  of  ground  water  reservoirs,  the  reclamation  of 
salt-affected  lands,  and  in  the  maintenance  of  a  favorable  salt 
balance  within  agricultural  soils.   Moreover,  as  water  costs  rise, 
more  accurate  knowledge  of  evapotranspiration  rates  will  become  of 
increasing  importance  in  order  to  achieve  greater  efficiencies  in 
irrigation  practices. 

Authorization 
Estimates  of  evapotranspiration  and  irrigation  require- 
ments have  long  been  a  part  of  water  development  investigations,  as 
conducted  by  the  department  and  its  predecessor  agencies.  The  preS' 
ent  program,  designed  to  provide  more  accurate  data  on  rates  of 
evapotranspiration,  was  initiated  in  July  195^  and  broadened  in 
1959.  pursuant  to  Senate  Bill  43^,  1959  Legislative  Session. 
Specific  authorization  for  conducting  these  studies  is  set  forth 

-2- 


in  Section  226  (e)  of  the  Water  Code,  which  states  that  the  depart- 
ment may  "Conduct  investigations  of  the  rate  of  use  of  water  for 
various  purposes  and  considering  various  soil  conditions." 

Objective 
The  overall  objective  of  the  vegetative  water  use  studies 
is  to  Investigate  and  establish  a  means  whereby  the  department  can 
accurately  determine  long-term  monthly  and  seasonal  irrigation  re- 
quirements and  evapotranspiration  for  the  principal  crops  grown 
within  the  various  agricultural  zones  throughout  California.   To 
accomplish  this  broad  objective,  the  vegetative  water  use  studies 
are  divided  into  three  principal  programs;  namely,  agroclimatic 
monitoring,  evapotranspiration  measurement  and  correlation,  and 
irrigation  requirement  determination.   The  first  two  of  these  pro- 
grams are  designed  to  accomplish  the  following  primary  objectives: 
first,  to  collect  agroclimatic  data  in  major  agricultural  areas  to 
provide  a  means  of  dividing  the  State  into  agroclimatic  zones  of 
potential  water  use,  and  for  estimating  evapotranspiration  within 
those  zones;  and  second,  to  test,  on  a  statewide  basis,  certain 
procedures  suggested  by  fundamental  research  by  the  University  of 
California  and  other  agencies,  regarding  correlation  of  evapotranspi- 
ration with  various  types  of  agroclimatic  data.  The  objective  of 
the  third  program  is  to  correlate  measured  values  of  total  applied 
water  with  evapotranspiration.  These  data  will  make  possible  the 
calculation  of  other  pertinent  water  use  information,  such  as  ir- 
rigation efficiencies  and  drainage  requirements.   Very  little  has 
been  accomplished  on  the  third  program  to  date, 

-3- 


Scope  of  Present  Program  and  Report 

To  accomplish  the  foregoing  objectives.  It  Is  necessary 
to  measure  evapotransplratlon  for  various  crops  within  the  major 
agricultural  zones  of  the  State,  and  to  measure  various  climatic, 
plant,  and  soil  factors  which  Influence  evapotransplratlon.   To 
date,  accurate  measurements  of  evaporation  have  been  made  of  only 
a  few  crops  within  certain  of  the  major  agricultural  service  areas 
of  the  State,  because  of  financial  and  personnel  limitations.  Ad- 
ditional installations  will  be  required  to  provide  complete  evalua- 
tion of  all  major  agricultural  zones  and  the  principal  crops  grown 
within  California. 

In  order  to  maximize  the  utility  of  the  data  provided  by 
the  relatively  few  evapotransplratlon  measurement  stations,  a  cor- 
relative program  has  been  carried  on  to  relate  evapotransplratlon 
to  evaporation  indices.   Theoretically,  coefficients  derived  by 
comparing  evapotransplratlon  to  evaporation  from  pans  or  atmometers 
can  be  used  to  make  reliable  estimates  of  evapotransplratlon  within 
any  agrocllmatic  zone  where  evaporation  data  are  available.   Basic 
research  on  such  relationships  is  being  conducted  by  the  University 
of  California  as  a  part  of  the  vegetative  water  use  program. 

The  agrocllmatic  monitoring  program,  described  fully  in 
Chapter  II,  is  designed  to  collect  the  basic  agrocllmatic  data 
necessary  to  make  reliable  estimates  of  evapotransplratlon  within 
each  agrocllmatic  zone.   Chapter  III  discusses  evapotransplratlon 
measurements  and  the  collection  of  data  relative  to  plant  condi- 
tions, soil  moisture,  and  other  factors  which  may  affect  evapo- 
transplratlon rates.  The  criteria,  methods,  and  Instrumentation 

-4- 


used  in  the  measurements  are  described  generally,  and  the  data  col- 
lected through  i960  are  summarized.   Since  the  initiation  of  this 
program  in  195^^  improvements  and  standardizations  within  the  pro- 
gram have  vastly  Improved  the  quality  of  the  data  collected,  such 
that  one  hesitates  to  compare  data  collected  In  196O  with  earlier 
years  of  records.   Consequently,  Judgment  was  exercised  in  sum- 
marizing certain  of  the  earlier  data. 

In  Chapter  IV,  measured  evapotranspiration  rates  described 
in  Chapter  III  are  correlated  with  pan  and  atmometer  evaporation 
data  which  were  collected  concurrently  at  the  evapotranspiration 
plots.   The  pan  and  atmometer  coefficients,  so  derived,  are  then 
applied  to  the  agroclimatic  data  to  estimate  evapotranspiration 
for  a  few  crops  throughout  much  of  the  northern  part  of  the  State. 
While  comparisons  are  made  with  the  values  published  In  Bulletin 
No,  2,  it  is  not  the  Intent  of  this  report  to  imply  a  question  as 
to  the  accuracy  of  previous  values  used  by  the  department.   In- 
stead, this  report  Is  Intended  to  Indicate  some  of  the  problems 
involved  In  the  collection  and  analysis  of  the  data  and,  to  the 
extent  of  the  data  collected,  to  show  tentative  values  that  may 
be  used  for  the  determination  of  water  requirements  for  certain 
crops. 

A  great  deal  of  the  basic  research  fundamental  to  this 
study  was  conducted  by  the  University  of  California  at  Davis,  both 
prior  to  and  since  the  initiation  of  this  program.   The  continuing 
counsel  and  guidance  provided  by  various  members  of  the  University 
of  California  have  been  of  Invaluable  assistance  In  the  develop- 
ment of  these  studies. 


CHAPTER  II.   AGROCLIMATIC  MONITORING  PROGRAM 

As  stated  In  Chapter  I,  the  objective  of  the  agrocllmatlc 
monitoring  program  is  to  collect  and  analyze  climatological  data 
throughout  the  various  agricultural  water  service  areas  within  the 
State.   The  analyses  of  these  data  will  accomplish  two  purposes. 
First,  they  will  enable  segregation  and  delineation  of  zones  or 
areas  with  similar  evaporation  potentials.   Secondly,  these  data 
will  provide  a  basis  for  estimating  evapotranspiration  rates  of 
various  crops  within  those  zones.   This  can  be  accomplished  by 
utilizing  coefficients  which  relate  measured  crop  evapotranspira- 
tion (to  be  discussed  in  Chapter  III)  to  agrocllmatlc  data.   The 
program  of  correlating  measured  evapotranspiration  to  various 
evaporative  indices,  such  as  evaporation  pans  and  atmometers,  is 
discussed  in  Chapter  IV. 

To  date,  agrocllmatlc  stations  have  been  established  at 
typical  locations  within  certain  of  the  major  inland  agricultural 
areas  in  the  central  and  northern  portions  of  the  State.  The  data 
collected  and  summarized  in  this  report  comprise  weekly  measure- 
ments of  evaporation  from  U,  S.  Weather  Bureau  Standard  Class  A 
pans,  and  differences  of  evaporation  between  Livingston  black  and 
white  atmometers.   Measurement  of  solar  radiation,  air  temperature, 
and  humidity  was  made  at  a  few  locations.   These  data,  however, 
are  not  included  in  this  report,  as  research  regarding  their  re- 
lationships to  evapotranspiration  and  methods  of  analysis  are  still 
in  the  process  of  development. 

As  of  i960,  the  program  Included  52  stations,  although 
a  total  of  112  stations  have  been  operated  for  various  periods  of 


time.   Many  of  the  original  stations  have  been  discontinued  be- 
cause of  unfavorable  site  conditions  or  other  causes.   The  location 
and  status  of  each  station  are  shown  on  Plate  1,  entitled  "General 
Locations  of  Agro climatic  Stations,  1954-60."  A  more  detailed  de- 
scription of  each  of  the  agroclimatlc  stations  is  presented  in 
Table  A-1  of  Appendix  A. 

Instrumentation  at  Agroclimatlc  Stations 
Two  types  of  equipment  were  utilized  to  measure  evapora- 
tion potential;  the  Livingston  spherical  atmometer,  and  the  U.  S, 
VJeather  Bureau  Standard  Class  A  evaporation  pan.   U.  S.  Forest 
Service  precipitation  gages,  approximately  8  Inches  in  diameter 
and  10,5  inches  in  height,  were  installed  at  all  agroclimatlc 
stations  at  the  same  elevation  above  ground  as  prescribed  for  a 
standard  U,  S,  Weather  Bureau  nonrecording  rain  gage.   Following 
is  a  description  of  evaporation  equipment  in  use  and  methods  of 
installation. 

A tmo meters 

A  Livingston  spherical  atmometer  is  a  specialized  instru- 
ment used  for  measurement  of  evaporation.   The  atmometer  is  a  hol- 
low porous  porcelain  sphere  5  centimeters  in  diameter.   In  a  typical 
assembly  the  sphere  is  mounted  on  a  1-gallon  water  supply  bottle 
by  means  of  a  small-diameter  glass  tube.  The  sphere  and  connecting 
tube  ar'e  filled  with  distilled  water,  with  the  lower  end  of  the  tube 
extending  nearly  to  the  bottom  of  the  reservoir  bottle.   Thus,  there 
is  a  continuous  water  system  from  the  reservoir  bottle  to  the  outer 
surface  of  the  porous  sphere,  where  evaporation  takes  place. 


Evaporation  Is  determined  by  measuring  the  amount  of  water  re- 
quired to  refill  the  reservoir  bottle  to  a  reference  mark.   A 
typical  atmometer  assembly  Is  shown  on  Figure  1, 

Atmometers  are  operated  as  pairs  consistlnc  of  one 
white  and  one  black  sphere  set  15  Inches  apart  and  ^4  Inches 
above  ground  surface.   Prior  to  1958^  many  installations  had  only 
a  single  pair  of  atmometers;  however,  since  that  time  three  or 
more  pairs  of  atmometers  have  been  Installed  at  each  of  the  sta- 
tions included  in  the  monitoring  program. 

Evaporation  Pans 

U,  S.  Weather  Bureau  Standard  Class  A  evaporation  pans 
were  adopted  in  the  agroclimatic  program  in  1937  and  installed 
at  certain  of  the  stations.  The  pans  were  installed  in  accordance 
with  the  procedure  prescribed  in  "instructions  for  Climatological 
Observers,"  Circular  B.  Tenth  Edition,  Revised  October  1955,  U.  S. 
Department  of  Commerce. 

All  stations  included  in  the  Agroclimatic  Monitoring 
Program  are  periodically  inspected  to  ascertain  that  equipment 
is  correctly  installed  and  properly  exposed.  Complete  records 
for  all  stations  are  available  in  the  files  of  the  department. 
Typical  agroclimatic  station  installations  are  shown  in  Figure  2. 

Agroclimatic  Data  Analysis 
Summaries  of  the  agroclimatic  data  collected  during  the 
period  from  July  195^  through  December  196O  are  shown  in  Tables  1 
and  2.   Table  1  shov;s  the  means  of  monthly  evaporation  from  standard 


-9- 


IBLACK    PORCELAIN    SPHERE! 


PORCELAIN    STEM 


LIVINGbl^,N    ATMOMbiEr 


RUBBER    STOPPER 


IGLASS    TUBE] 


P\'^'    VtNT  TUBEI 


RUBBER  STOPPER! 


jU'I^TlN    plusI 


Figure    I,   ATMOMETER    ASSEMBLY 


•..ir%A:jsl-^.>  ,fi^. .  >-#■ 


station  Located 

in  Irrigated  Pasture 

near  Lodi 


"> 


* 


S  -T 


Station  Located 

in  Dryland 

Environment 

near  Redding 


.s'^-..^-  >'*-A.*':-  ••  •  "i-^-'"  '-;f^'  'i:-: 


•  1  t^  li* 

^-U   i 


?k'..>: 


f 


Station  Located 
in  Non- irrigated 
Alfalfa  near 
Adin,  Modoc  County 


FIGURE  2.  TYPICAL  AGROCLIMATIC  STATIONS 


TABLE   1 

MEAH  MOKTHLY  EVAPORAHOB 

FROM 

STANDARD 

U. 

S.     WEATHER  BUREAU  EVAPORATION  PASS 

(In 

Years 

May 

Qivlronment  and  area 

:  Station 

of 
Reconl 

Sept. 

Total 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Ort. 

■    Nov. 

Dec-. 

Pasture 

KLamath-Trinlty  Mt.  Valleya 

2 

1959-60 

6.95 

8.76 

11.06 

8.1.1. 

6.18 

41.39 

Sacramentolttver  Basin  Mountain  VaUeys 

9 

1957-60 

1.1.8 

3.25 

5.10 

6.16 

7.69 

b.96 

8.78 

6.16 

3.86 

1.66 

0.66 

37.75 

Sacramento  RLver  Basin  Fbothllls 

1957-60 

1.52 

2.29 

3.56 

5.19 

6.10 

9.15 

10.66 

9.27 

6.1.4 

5.00 

2.20 

1.52 

41.82 

Sacramento  River  Basin  Valley  Floor 

12 

1958-60 

1.65 

2.1.9 

1..01. 

5.1.8 

7.26 

10.28 

10.73 

9.18 

6.87 

5.34 

2.58 

1.74 

44.32 

Son  Joaquin  River  Basin  Valley  Floor 

11 

1959-60 

1.67 

1..19 

6.08 

8.84 

10.60 

10.55 

9.08 

6.76 

5.14 

1.92 

1.30 

tv.u 

Tiili^T^    T«kp    Rflflin   Vnllpy    Plnnr 

6 

1958-60 

1.79 

h.li 

5.76 

8.77 

9.71. 

9.36 

6.00 

4.24 

1.96 

lassen-Alplne  Mountain  Valleys 

6 

1957-60 

6.30 

8.91 

lo.grr 

9.81 

6.85 

4.35 

- 

42.84 

Dryland 

SacramentoRlver  Basin  Mountain  Valleys 
SacramentoRlver  Basin  Foothills 
SacramentoRlver  Basin  Valley  Floor 


1958-60  —  1.20     2.99 

7  1958-60       1.42       2.75 

9  1958-60       1.26       2.48 


5.98  5.95  10.02  12.03  11.06  7.41 
6.52  8.69  13.36  15.04  12.46  10.27 
6.52   8.95  13.25  14.03  11.87   9.45 


2.09  -  46.47 
3.89  2.94  59.82 
3.19  1.90   57.55 


(in  milliliters) 


:  Number 

:     of 

:  Stations 

Years 
Record 

HOirrHS 

S^t. 
total 

Environment  and  area 

Jan. 

Feb. 

Mar. 

Apr. 

May 

.June       Ju}y    Aug. 

Sejt. 

Oct. 

Nov. 

Dec. 

SacramentoRlver  Basin  Mountain  Valleys 
Sacramento  River  Basin  Foothills 
Sacramento  River  BasinValley  Floor 
San  Joaquin  RLver  Basin  Valley  Floor 
Tulare  Lake  Basin  Valley  Floor 
Lassen-Alpine  Mountain  Valleys 


-Trinity  Mountain  Valleys  3 

Sacramento  River  Basin  Mountain  Valleys  11 

Sacramento  River  Basin  Valley  Floor  1? 

San  Joaquin  River  Basin  Valley  Floor  I3 

Tulare  laJte  Basin  Valley  Floor  9 


Klamath- Trinity  Mountain  Valleys 
Sacramento  River  Basin  Mountain  Valleys 
Sacramento  River  Basin  Foothills 
Sacramento  River  Basin  Valley  Floor 
lassen-Alpine  Mountain  Valleys 


MisceUaneous 

River 


sin  Veilley  Floor 


1955-60 
1958-60 
1958-60 
1959-60 
1958-60 
1955-60 

292 
324 
374 

1955 

1955-59 

1955,58-60 

1958-60 

1958-60 

it 
402 

1954-60 
1954-60 
1959-60 
1954-60 

1955-56  s, 

1958-59 

in 

1954-55, 

57,  4  60 

-12- 


494 

572 

619 

491 

588 

6l4 

529 

569 

580 

520 

572 

580 

460 

545 

572 

550 

558 

486 

537 

566 

539 

580 

618 

470        548      571        582      548 
462        538      589        617       563 


521 

584 

546 

413 

536 

569 

540 

408 

310 

7?6 

658 

593 

458 

388 

588 

655 

573 

465 

366 

582 

535 

2765 

?703 
2754 


2510 
2523 
?753 
2?92 


U,  S,  Weather  Bureau  pans.   Table  2  indicates  the  mean  monthly 
difference  of  evaporation  between  Livingston  spherical  black 
and  white  atmometers. 

At  the  Initiation  of  the  proo'ram  in  195^j  little  was 
known  of  the  effects  of  the  immediate  ground  cover  environment 
on  evaporation  from  atmometers  and  pans.   Furthermore,  little 
consideration  had  ever  been  given  to  the  effects  on  evaporation 
rates  of  surrounding  land  areas  or  cleanliness  of  pans  at  sta- 
tions having  apparently  similar  immediate  environmental  conditions. 
In  analyzing  the  data  it  became  apparent  that  certain  of  these 
factors  are  extremely  Important. 

In  the  initial  tabulations  of  evaporation  data,  great 
differences  were  noted  between  adjacent  stations  having  dissimilar 
environmental  conditions.   A  tabulation  on  the  basis  of  station 
environment  shows  this  to  be  especially  true  for  evaporation  pans, 
as  may  be  noted  in  Table  1,   For  example.  Table  1  indicates  that 
the  May  through  September  total  of  the  mean  monthly  evaporation 
from  pans  located  on  dry-farmed  rangelands  was  more  than  25  per- 
cent greater  than  evaporation  from  pans  situated  on  irrigated 
pasture.   This  difference  became  increasingly  greater  during  the 
summer  months.   The  higher  and  increasingly  greater  evaporation 
on  dry-farmed  rangelands  resulted  from  the  greater  availability 
of  energy  in  surrounding  dryland  areas,  and  the  increase  of  ad- 
vectlve  heating  that  results  as  the  drylands  exhaust  moisture 
carried  over  from  wintertime  precipitation  during  the  summer. 


-13- 


An  interesting  fact  determined  from  studies  at  the 
Bakersfield  station  was  that  cleanliness,  or  presence  of  algae 
growth,  had  little  effect  upon  evaporation  rates  from  evaporation 
pans.   During  an  l8-month  period  starting  in  January  1959,  three 
pans  were  maintained  in  the  same  environment  and  were  treated  in 
an  identical  manner,  except  that  algae  was  permitted  to  grow  in 
one  pan  while  the  other  two  were  cleaned  frequently.   The  dif- 
ference of  evaporation  vms  small,  with  only  3  percent  greater 
evaporation  in  the  pan  where  algae  was  allowed  to  grow. 

In  an  evaporation  investigation  carried  on  by  A,  A.  Youn 
in  Southern  California  during  the  period  from  1935  to  1939,  inclu- 
sive, a  study  was  conducted  to  determine  the  effect  of  pan  color 
upon  evaporation.   He  found  differences  varying  from  approximately 
17  percent  less  to  7  percent  more  than  from  a  standard  U.  S,  Weathe 
Bureau  pan.   It  is  of  Interest  to  note  that  evaporation  from  a 
dark  green  colored  pan  was  2.5  percent  greater  than  that  from  the 
standard  U,  S,  Weather  Bureau  pan.   The  presence  and  growth  of 
algae  appear  to  give  similar  results. 

The  difference  in  evaporation  between  black  and  white 
atmometers,  as  shown  in  Table  2,  appears  to  be  affected  less  by 
environmental  conditions  than  are  pans.   This  indicates  a  differenc 
in  response  bet^^reen  pans  and  atmometers  to  various  climatic  condi- 
tions.  This  will  be  discussed  further 3n  Chapter  IV. 

Monthly  evaporation  data  from  pans  and  atmometers  for 
each  year  and  for  each  station  are  set  forth  in  Tables  A-2  and 
A-3,  respectively,  of  Appendix  A,  The  data  are  segregated  by  area 
and  by  environment. 

-14- 


The  area  designations  set  forth  in  this  report  are 
arbitrary  andj  in  general,  principally  geographical  subdivisions. 
When  additional  years  of  data  become  available,  these  area  break- 
downs must  be  reconsidered.   Analysis  of  the  records  of  individual 
stations  to  date  indicates  as  much  variability  in  evaporation  be- 
tween adjacent  stations,  within  any  one  area,  as  between  areas. 
This  variability  is  shoim  in  Tables  3  and  4,  in  which  all  of  the 
stations  located  on  irrigated  pasture  in  1959  and  196O  were  arranged 
in  order  of  decreasing  evaporation  rate  by  month.  The  same  was  done 
for  the  1959  and  196O  dryland  stations.   On  the  basis  of  these  data. 
It  is  concluded  that  no  definite  segregation  of  the  stations  into 
areas  of  uniform  evaporation  Is  possible. 

A  general  pattern  has  been  discerned  with  certain  of  the 
stations  tending  to  be  high  and  others  low.   There  are  indications 
that,  for  stations  having  similar  environments  immediately  sur- 
rounding the  site,  adjacent  dryland  areas  exert  climatic  influences 
and  affect  evaporation  rates  at  the  station  site. 

This  factor  is  being  given  further  consideration  In  rela- 
tion to  the  agrocllmatlc  stations  currently  in  operation.   Efforts 
are  being  made  to  standardize  conditions  where  pan  and  atmometer 
data  are  collected.   Insofar  as  possible,  large,  well-irrigated 
pastures  providing  nearly  100  percent  ground  cover  are  being  se- 
lected as  sites  for  agrocllmatlc  stations.  As  data  are  obtained 
under  similar  environmental  conditions,  more  conclusive  compari- 
sons may  be  made.   It  may  be  fo\jnd  that  there  are  small  differences 
In  monthly  evaporative  rates  between  different  agricultural  areas 
of  the  State,  and  that  the  length  of  growing  season  is  the  most  im- 
portant factor  affecting  seasonal  evapotranspiration  in  Inland  areas. 

-15- 


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19 


CHAPTER  III.   EVAPOTRANSPIRATION  MEASUREMENT 

The  objective  of  the  evapotransplration  measurement  and 
correlation  program  Is  to  provide  a  more  accurate  basis  for  pre- 
dicting evapotransplration  for  the  major  crops  In  the  various  agri- 
cultural areas  of  the  State.  This  is  to  be  accomplished  through 
measurements  of  evapotransplration  of  various  crops  at  several  in- 
land locations  having  different  climatic  conditions,  and  correlating 
with  the  evaporative  demand,  as  measured  by  evaporation  pans  and 
atmometers.   This  chapter  discusses  the  techniques  and  procedures 
utilized  in  the  measurement  of  evapotransplration,  and  changes  that 
have  occurred  during  the  development  of  the  study.   In  Chapter  IV 
the  correlation  of  the  evapotransplration  with  pan  and  atmometer 
evaporation  data  will  be  discussed  and  analyzed. 

The  principal  evapotransplration  stations  are  located 
near  Bakersfield  in  the  southern  San  Joaquin  Valley  and  near  Al- 
turas  and  Fall  River  Mills  in  mountain  valleys  of  the  Sacramento 
River  Basin.   Plate  2,  entitled  "General  Location  of  Evapotranspl- 
ration Stations,  1955-1960,"  shows  the  location,  type,  and  status 
of  each  station.   More  detailed  information  is  given  in  Tables  A-4 
and  A-5  of  Appendix  A,  Measurements  were  made  primarily  on  alfalfa 
and  grass,  which  are  grown  universally  throughout  the  State.  As 
plant  and  soil  moisture  conditions  affect  evapotransplration  rates, 
evaluation  of  these  factors  is  also  an  essential  part  of  evapo- 
transplration measurement. 


-21- 


Measurement  of  Data  Related  to  Evapotranspiratlon 

Correlative  pan  and  atmometer  evaporation  measurements 
were  made  at  agroclimatic  stations  established  near  the  evapotran- 
spiratlon measuring  stations.   These  stations  are  listed  in  Table  ^ 
of  Appendix  A,   Detailed  information  regarding  the  agroclimatic  sta 
tions,  and  pan  and  atmometer  data,  are  given  in  Tables  A-1,  A-2,  and 
A-3  of  that  appendix. 

At  Arvin  (in  Kern  County)  the  pan  and  atmometer  data  were 
initially  collected  at  stations  (Arvin  Jewett  #1  and  #2)  located 
in  an  irrigated  alfalfa  field  near  the  evapotranspiratlon  station, 
or  soil  moisture  depletion  plots.   In  June  1959^  a  new  station 
(Arvin-Frick)  was  established  in  an  irrigated  grass  environment. 
All  of  the  soil  moisture  depletion  plots  were  within  1  mile  of 
this  station. 

Only  atmometer  data  were  collected  at  the  Pittville  AA 
plot  site  (in  eastern  Shasta  County)  during  1959.   This  agroclimat 
station  is  identified  as  Pittville  1  S.   Pan  data  were  collected 
within  an  irrigated  pasture  site,  designated  as  Fall  River  Mills 
4  Nw,  during  1959:»  and  until  June  I960,  but  due  to  unfavorable 
operational  procedures  at  this  site  in  1959^  the  pan  data  were 
not  used  in  this  report. 

In  June  I960,  an  agroclimatic  station  vms  established 

at  a  location  within  an  irrigated  pasture  8  miles  west  of  the 

Pittville  AA  plot.   This  station  is  identified  as  Glenburn  DWR. 

Comparison  of  atmometer  evaporation  measurements  at  the  Pittville 

1  S  and  the  Glenburn  DWR  stations  showed  that  the  difference  in 

evaporation  between  the  black  and  white  atmometers  is  very  close 

for  these  two  locations. 

-22- 


At  the  Arvln  and  Glenburn  sites,  three  sets  of  nev;  at- 
mometers  were  Installed  at  the  berlnning  of  each  season,  and  each 
month  one  pair  was  replaced  with  a  new  pair.   At  Arvln,  three  pans 
were  operated,  and  the  evaporation,  which  v;as  nearly  the  same,  was 
avera,";ed. 

Data  on  percent  of  r;round  cover  were  collected  to  deter- 
mine effects  of  varying  cover  on  evapotranspiratlon  rates.   The 
term  "percent  ground  cover,"  as  used  in  this  report,  refers  to  the 
percentage  of  ground  surface  covered  by  a  canopy  of  living  foliage 
as  viewed  looking  downward  from  directly  above  the  crop.   During 
the  first  years,  1955-1937,  few  records  were  kept  of  percent  ground 
cover.   However,  from  1958  through  1960  it  was  standard  procedure 
to  measure  crop  height,  estimate  percent  ground  cover,  and  record 
both. 

When  most  of  the  moisture  which  plants  can  readily  ex- 
tract from  within  the  root  zone  has  been  used,  crop  growth  is 
slowed  and  evapotranspiratlon  rates  may  also  be  correspondingly 
affected.   To  estimate  available  soil  moisture  at  the  test  plots, 
samples  were  taken  and  laboratory  measurements  of  the  moisture 
content  of  the  soil  were  made,  utilizing  the  pressure  plate  mem- 
brane technique  with  pressures  varying  from  0.1  to  15  atmospheres. 
T  ensiometers,  instruments  ^^^hich  can  be  used  to  measure  availability 
of  soil  moisture  for  crop  utilization,  were  installed  at  some  plots. 
Calculations  of  available  soil  moisture  in  the  root  zone  were  based 
on  the  difference  between  moisture  profiles  determined  from  field 
measurements  and  moisture  profiles  representing  the  moisture  level 
below  which  crops  cannot  readily  extract  moisture. 


■23- 


Criteria  for  Selection  of  Plots 
In  the  selection  of  plots  for  the  measurement  of  evapo- 
transplration^,  certain  physical  conditions  are  recognized  as  es- 
sential to  collecting  valid  data.   Experience  over  the  years  has 
emphasized  the  Importance  of  certain  necessary  conditions.   Un- 
fortunately,  the  most  ideal  plot  conditions  are  difficult  to  find, 
and  considerable  time  and  effort  have  been  expended  over  the  years 
selecting  the  most  favorable  sites.   However,  this  is  not  to  imply 
that  evapotranspiration  rates  would  necessarily  be  different  under 
different  conditions.   The  following  criteria  indicate  the  condi- 
tions under  which  good  measurements  of  evapotranspiration  repre- 
sentative of  field  conditions  can  be  obtained.  After  good 
measurements  have  been  obtained  under  these  conditions,  the  studie; 
should  be  broadened  to  include  some  of  the  adverse  soil  and  other 
conditions  which  might  affect  evapotranspiration. 

1.  Measurement  sites  should  be  located  200  feet  inside 
the  edge  of  the  irrigated  field  to  avoid  accentuated  border  effect; 

2.  Generally,  the  land  should  be  of  smooth  topography. 

3.  Since  the  sites  are  located  on  private  lands,  it 
is  necessary  to  have  freedom  of  access  and  cooperation  of  the 
landowner  or  manager, 

4.  The  soil  should  be  deep,  well  drained,  productive, 
and  unaffected  by  salinity.  The  soil  preferably  should  also  be 
medium  textured,  as  very  fine  or  very  coarse  textures  have  un- 
favorable soil-moisture  relationships.  The  soil  profile  should 
not  be  stratified  to  such  an  extent  as  to  impede  moisture  flow 
or  create  sampling  pi'-oblems. 


•  24- 


5.  The  lrrln:ated  field  should  be  located  in  typical 
lrri,e^ated  areas,  not  on  the  frlnee  of  irrigated  areas. 

6.  There  should  be  an  adequate  supply  of  irrigation 
v/ater.   It  is  highly  desirable  that  there  be  possibilities  for 
controllinr;  and  measurin,";  the  amount  of  vvater  applied  to  the  test 
plot. 

7.  Except  for  measurements  which  are  made  by  evapo- 
transplrometers,  no  water  table  should  exist  within  or  near  the 
root  zone  of  the  crop, 

Evapotranspiration  Measurement  Techniques  and 
Discussion  of  Development  and  Current  Methods 

The  tools  and  techniques  used  in  this  study  to  measure 

evapotranspiration  fall  into  two  general  categories.   One  is  field 

plot  sampling,  and  the  other  is  evapotranspirometer  measurements. 

Field  Plot  Sampling  -  Gravimetric  Method 

Periodic  measurement  of  soil  moisture  provides  a  means 
of  determining  total  change  of  water  content  within  a  selected 
portion  of  the  soil  profile.   Evapotranspiration  may  be  determined 
from  data  on  soil  moisture  change  and  precipitation.   Soil  moisture 
must  be  sampled  or  measured  each  time  at  or  near  the  same  location 
in  each  plot,  with  several  locations  being  situated  in  each  plot. 
Moreover,  the  moisture  determinations  must  be  made  at  least  twice 
following  wetting  of  the  soil  by  any  heavy  irrigation  or  heavy 
precipitation.   To  obtain  satisfactory  results,  it  is  necessary 
that  sufficient  time  lapse  be  permitted  following  thorough  wetting 
of  the  soil  (usually  several  days)  before  making  the  first  moisture 


•25- 


determination  in  a  cycle  of  measurements.   Otherwise,  moisture 
moving  out  of  the  sampled  profile  would  be  incorrectly  included 
as  evapotranspiration  in  the  soil  moisture  depletion  measurement. 

During  the  growing  season,  the  general  procedure  was  to 
sample  approximately  every  seven  days,  except  as  modified  by  ir- 
rigation, harvest,  or  other  cultural  (farming)  operations.   Durin: 
the  nongrowing  season,  measurements  were  made  less  frequently  be- 
cause of  the  lower  rates  of  v/ater  use. 

At  the  initiation  of  the  evapotranspiration  measuring 
program  in  1955,  the  gravimetric  technique  was  accepted  as  the 
best  method  available,  and  was  the  first  technique  employed  in 
the  studies  reported  here.   Moisture  content  of  soil  samples  was 
determined  by  weight  change  resulting  from  moisture  loss  during 
oven  drying.   Soil  samples  were  taken  by  means  of  a  soil  tube,  in 
1-foot  increments  of  depth,  from  the  soil  surface  to  a  depth  of 
7  or  9  feet.   As  the  soil  tube  is  difficult  to  handle  at  depth 
below  9  feet,  sampling  below  that  depth  was  attempted  only  in 
special  cases  where  knowledge  of  the  substratum  conditions  was 
desired. 

The  initial  evapotranspiration  measurements  were  made 
in  the  mountain  valley  areas  in  the  northern  and  northeastern  part 
of  the  State,  and  in  the  northern  Sacramento  Valley.   The  objectiv 
at  that  time  was  to  determine  the  irrigation  requirements  of  only 
those  areas.   Plots  in  the  mountain  valleys  were  located  on  typica 
irrigated  parcels  of  land.   The  irrigated  lands  in  this  area  exist 
as  narrow  and  isolated  "oases"  separated  by  large  areas  of  native 
vegetation. 

-26- 


From  three  to  eight  core  holes  were  made  per  sampling. 
This  number  did  not  prove  to  be  adequate  because  of  Inherent 
variability  of  the  soils. 

During  analysis  of  data  collected  during  the  195:?  season, 
it  was  determined  that  observation  holes  should  have  been  main- 
tained at  all  plots  to  determine  if  water  table  conditions  existed. 
Through  observation  holes  on  a  few  of  the  plots,  and  examination 
of  soil  samples  taken  from  the  lower  profiles,  it  was  found  that 
water  tables  did  exist  on  some  plots  where  they  were  not  expected. 
When  a  water  table  is  present  in  or  near  the  root  zone,  there  is 
a  probability  that  the  crop  will  utilize  some  of  this  source  of 
moisture.   It  is  impossible  to  determine  this  amount. 

The  greatest  problem,  however,  was  that  irrigation  in 
some  cases  added  too  much  water,  and  in  other  cases  was  too  in- 
frequent or  too  little.   As  previously  mentioned,  when  too  much 
water  is  applied,  downward  moisture  movement  continues  for  a  con- 
siderable length  of  time.   A  series  of  field  moisture  measurements 
will  include  both  moisture  movement,  or  change,  due  to  plant  ex- 
traction and  evaporation.   If  too  little  water  is  applied,  the 
soil  moisture  may  become  critically  short,  and  crop  growth  may 
be  affected.   If  the  soils  become  very  dry,  the  evapotranspira- 
tion  rate  may  also  be  affected. 

For  the  next  several  seasons,  work  was  concentrated  on 
fewer  plots,  and  more  detailed  observations  were  made  of  crop 
growth,  presence  of  water  tables,  and  other  conditions.   As  the 
need  for  irrigation  control  became  recognized  as  being  critical 


-27- 


to  obtain  adequate  evapotransplratlon  data  from  soil  moisture  de- 
pletion measurements,  attempts  to  modify  Irrigation  were  initiated. 

It  was  observed  that  weekly  visits  to  plot  sites  ad- 
verseley  affected  the  crop  cover  and  soil  conditions  by  trampling 
the  crop.   To  overcome  these  undesirable  effects,  a  portable 
sampling  platform  v/as  built  in  1956  to  sample  one  of  the  plots. 
This  was  the  forerunner  of  platforms  which  were  used  later  with 
neutron  probes. 

In  1957^  the  water  use  studies  were  expanded  to  other 
areas  of  the  State,  Alfalfa  fields  were  sampled  in  Stanislaus 
and  Kings  Counties.   These  plots  were  abandoned  because  data  ob- 
tained from  them  were  not  considered  reliable  for  estimating  evapo- 
transplratlon because  of  excessive  moisture  movement  that  resulted 
from  overirrigation  at  the  Stanislaus  County  plots  and  unfavorable 
soil  conditions  at  the  plots  in  Kings  County. 

In  1958,  one  man  was  stationed  in  Kern  County  following 
a  reconnaissance  for  plot  sites.   Plots  of  alfalfa,  grapes,  and 
plums  were  sampled.   Problems  of  two  kinds  were  encountered.   On 
plots  receiving  lesser  quantities  of  irrigation,  the  crops  ex- 
tracted moisture  from  below  the  zone  sampled,  while  on  plots  re- 
ceiving very  frequent  irrigations,  considerable  moisture  movement 
occurred  between  sampling. 

No  further  gravimetric  samples  were  taken  following  the 
adoption  of  neutron  scattering  equipment  in  the  spring;  of  1959. 
While  complete  detailed  records  were  kept  and  calculations  made 
for  each  of  the  gravimetric  sampling  sites,  the  results  of  these 
measurements  are  not  Included  in  this  report, 

-28- 


Field  Plot  Sampling  -  Neutron  Scattering  Technique 

A  recently  developed  method  to  obtain  ttie  soil  moisture 
data,  referred  to  as  the  neutron  scattering  technique.  Is  based 
upon  the  principle  that  high  energy  or  "fast"  neutrons  are  mod- 
erated, or  "slowed  down,"  in  soils  almost  exclusively  by  hydrogen 
atoms  contained  in  soil  moisture.   The  instrument  consists  of  a 
source  of  "fast"  neutrons,  a  detector  tube  which  Is  sensitive 
only  to  "slow"  neutrons,  and  a  slow  neutron  counter.   Both  source 
and  detector  are  combined  in  a  cylindrical  probe  1.5  Inches  in 
diameter  by  l4  Inches  long.   The  probe  is  lowered  Into  the  soil 
through  a  small-diameter,  cased  hole  to  the  desired  depth,  sus- 
pended by  its  electrical  cable.  The  cable  is  connected  to  the 
counting  device  which  counts  pulses  produced  by  slow  neutrons 
returning  to  the  detector.   Since  the  "fast"  neutron  output  of 
the  source  is  essentially  constant,  the  count  recorded  in  a  fixed 
time  period  may  be  used  with  a  suitable  calibration  to  determine 
the  moisture  content  In  the  soil  surrounding  the  probe. 

The  neutron  scattering  technique  has  certain  advantages 
over  the  gravimetric  technique.   In  addition  to  the  ease  of  making 
deeper  measurements,  the  neutron  measurements  take  less  time,  re- 
peatedly represent  approximately  the  same  soil  mass,  and  are  gen- 
erally more  precise  than  gravimetric  measurements.   Measurement  of 
the  same  soil  mass  is  particularly  important,  since  soil  moisture 
distribution  and  extraction  patterns  appear  to  be  nonuniform.   It 
must  be  noted,  however,  that  overirrlgation  and  resulting  moisture 
movement  remain  a  problem  with  this  method.  Also,  for  greater 


.29- 


accuracy,  measurements  of  the  soil  surface  layer,  to  a  depth  of 
about  1  foot,  require  a  different  calibration  than  the  measure- 
ments at  greater  depth  below  the  soil  surface.   Determination  of 
a  suitable  calibration  is  under  study  by  the  department  and  other 
agencies  at  this  time.   It  is  believed  at  the  present  that  the  er^ 
ror  of  measuring  the  losses  of  water  from  the  soil  surface  is  not 
large,  considering  the  total  water  use  from  the  entire  profile, 
in  the  case  of  deeper  rooted  crops. 

Inherent  variabilities,  such  as  found  in  physical  measure 
ments  of  any  natural  phenomenon,  occur  in  soil  moisture  depletion 
measurements.  Generally,  although  affecting  any  given  measurement, 
such  variations  tend  to  be  compensating  and,  over  a  period  of  time, 
such  as  a  year,  tend  to  cancel  out. 

Two  neutron  scattering  devices  were  acquired  in  1958^ 
shortly  after  this  equipment  became  commercially  available.   The 
neutron  equipment  was  used  for  determination  of  soil  moisture  in 
all  field  plots  since  early  spring  of  1959.   The  same  criteria 
used  for  selection  of  gravimetric  sampling  plots  were  followed  in 
establishing  the  plots  sampled  with  the  neutron  probe. 

Effort  was  made  to  keep  the  crop  in  the  plot  area  gen- 
erally typical  of  the  normal  conditions  of  the  entire  field.   Light 
weight,  portable  sampling  platforms  with  working  areas  of  15  to  30 
square  feet  were  fabricated  in  1959  to  carry  the  neutron  scattering 
equipment.   These  also  served  as  portable  working  platforms.   They 
have  been  particularly  advantageous  in  facilitating  the  field  work 
and  in  avoiding  trampling  and  injury  to  the  alfalfa  and  grass  crops 


■30- 


and  compaction  of  the  soils.   Three  types  of  portable  sampling 
platforms  used  at  Fall  River  Mills  and  Bakersfield  are  shovm  in 
Figure  3. 

To  provide  neutron  probe  access  into  the  soil,  thin- 
walled  aluminum  tubes  20  feet  in  length  with  removable  l8-inch 
extensions  at  the  top  were  permanently  installed  flush  with  the 
soil  surface.   Stoppers  were  placed  in  the  tube  at  the  surface 
and  immediately  below  the  extension  to  exclude  foreign  material 
from  the  tubes.   In  this  way,  the  tubes  did  not  extend  above  the 
ground  to  Interfere  with  tillage  and  crop  cultural  operations. 
When  tillage  operations  damaged  the  upper  extensions,  they  were 
simply  and  easily  replaced.   The  access  tube  design  is  shown  in 
Figure  4. 

Plttvllle  Neutron  Probe  Moisture  Depletion  Measurements. 
The  Plttvllle  site  is  located  at  an  elevation  of  about  3^340  feet, 
in  the  northeastern  intermountaln  region,  at  a  latitude  of  4l  de- 
grees.  Selection  of  the  neutron  measurement  site  was  preceded  by 
four  years  of  gravimetric  sampling  in  the  Fall  River  Valley  and 
other  mountain  valleys  in  the  northeastern  area.   The  Plttvllle 
1  S  site  was  sampled  using  the  gravimetric  technique  in  1956,  1937 i 
and  1958.   This  prior  experience  indicated  that  the  Plttvllle  al- 
falfa field  possessed  the  desirable  combination  of  soil  and  irri.- 
gatlon  conditions  for  a  moisture  depletion  plot.   Topographically, 
the  site  is  gently  sloping  with  small  swales.   There  is  a  small 
ridge  6OO  feet  north  of  the  plot  site,  which  is  about  100  feet 
higher  than  the  plot.   The  land  at  the  plot  site  slopes  3  percent 


•31- 


Platform  Developed 
in  Early  Stage  of 
Program  for  Obtaining 
Soil  Cores 


Small,  Wheeled 

Platform  Used  to 

Measure  Soil 

Moisture  Depletion 

by  the  Neutron 

Scattering  Technique 


Aluminum  Platform 
Used  with  the 
Neutron  Scattering 
Equipment 


FIGURE  3.  PLATFORMS  USED  TO  MINIMIZE  CROP  DAMAGE  AND  SOIL  COMPACTION 


^ 


V/ 


1 


i 


No.9  Rubber    Stopper 


18"-  |5/8" Aluminum   Tubing 
(Top  Flush  With  Soil  Surface] 


2"-l'/2"  Plastic  Sleeve  To  Join  Tubing 
(Heated  To  Install) 


No.8  Rubber  Stopper 
(With  Hook  For  Removing) 


20'  -I  Vs"  Aluminum  Tubing 
Bottom  End  Capped 


Figure   4,   ACCESS    TUBE    DESIGN 


to  the  south-southwest,  and  the  30  acres  of  alfalfa  In  the  field 
are  surrounded  by  small  irrigated  fields,  dry- farmed  grain,  and 
native  vegetation.   Prevailing  winds  in  the  area  are  from  the  west 

Initially,  three  rows  of  five  access  tubes  were  installec 
73  feet  apart,  with  the  tubes  spaced  in  the  rows  15  feet  apart. 
In  September  1959^  four  more  tybes  were  installed  in  one  of  the 
rov;s,  and  the  other  two  rows  abandoned,  reducing  the  plot  to  nine 
tubes.   This  enabled  the  plot  to  be  irrigated  in  two  days,  rather 
than  the  three  to  four  days  required  for  the  sprinklers  to  pass 
over  the  original  three  rows  of  access  tubes. 

Irrigation  water  is  applied  by  a  portable  sprinkler 
system,  using  full  circle  (360  degrees)  rotating  sprinklers.   The 
sprinklers  sometimes  stuck  in  one  position,  and  irrigation  applica 
tion,  as  a  result,  was  not  uniform  enough  to  determine  applied  wat< 
from  pumping  records.   This  plot  was  subjected  to  somewhat  deficit 
irrigation,  which  left  a  dry  zone  generally  below  a  depth  of  8  fee' 
For  this  reason,  the  soil  moisture  measurements  can  be  used  with 
confidence  as  estimate  of  evapotranspiration. 

Neutron  moisture  depletion  measurements  were  made  during 
1959  and  196O  at  another  alfalfa  site  3  miles  west  of  the  Pittvill«' 
plot.  Due  to  apparent  excessive  moisture  movement,  however,  the  rt 
suits  of  these  measurements  are  not  included  in  the  report. 

Arvin  Neutron  Probe  Moisture  Depletion  Measurements. 
These  measurement  sites  are  in  the  southern  San  Joaquin  Valley, 
near  the  35  degree  latitude,  located  at  an  elevation  of  about  hkO 
feet.   The  plot  sites  are  on  broad,  smooth,  recently  formed  fans 


-34- 


from  the  outwash  of  the  Sierra  Nevada  Ranre  at  the  southern  end  of 
the  valley.  The  land  slopes  to  the  southwest  at  the  plot  area  at 
about  30  feet  per  mile  (0.6  percent). 

Irrigation  in  the  area  is  supplied  from  deep  wells 
lifting  water  several  hundred  feet.   All  of  the  Arvin  plots  are 
located  on  Hesperia  fine,  sandy  loam.   This  soil  has  no  apparent 
clay  or  cemented  layers.  Moisture  drainage  is  good,   Noncontinuous 
silt  layers  and  pockets  of  silt  of  varying  thickness  are  found 
from  3  feet  down  to  22  feet  below  the  surface.   Plot  sites  were 
located  where  the  least  amount  of  silt  layers  are  found.   Sur- 
rounding the  sampling  areas  were  irrigated  orchards,  vineyards, 
alfalfa,  cotton,  and  other  crops.   The  irrigated  area  extends  20 
miles  to  the  north,  15  miles  to  the  east,  kO   miles  to  the  south, 
and  60  miles  to  the  west. 

Four  crops,  cotton,  alfalfa,  plum  orchard,  and  fescue 
grass,  were  sampled.   All  sites  were  irrigated  by  furrow  or  border 
methods.   In  order  to  obtain  reasonably  precise  data,  more  than 
20  sampling  tubes  were  installed  on  the  cotton  and  alfalfa.   Six 
tubes  each  were  installed  on  the  plums  and  grass  plots  for  ex- 
ploratory purposes,  the  intent  being  to  determine  moisture  extrac- 
tion patterns. 

The  plum  orchard  is  planted  on  a  24-foot  square  pattern. 
Water  is  applied  to  five  or  six  straight  furrows  running  in  one 
direction.   Results  of  the  neutron  probe  measurements  indicate  that 
the  extraction  of  moisture  is  greatest  from  the  furrov/  area  near 
the  trees,  intermediate  from  the  middle  furrows,  and  least  from  the 
soil  in  the  tree  rov;s.   Extraction  was  noted  to  a  depth  of  l6  feet. 
Depth  of  extraction  probably  depends  largely  on  Irrigation  practices, 

-35- 


On  the  grass  plot,  the  moisture  was  extracted  primarily 
from  the  upper  2  or  3  feet.   With  such  a  large  portion  of  the  total 
water  use  from  such  shallov/  depths,  the  inherent  uncertainty  of 
surface  neutron  probe  measurements  assumes  greater  importance.   It 
has  been  concluded  that  the  neutron  scattering  technique  is  not  well- 
suited  for  measuring  evapo trans pi rat ion  of  grasses  due  to  their 
shallow  moisture  extraction  patterns  and  frequent  irrigations.   Plan 
have  been  made  to  use  evapotranspirometers  on  this  crop. 

On  the  alfalfa  plot,  ample  tubes  were  sampled  to  obtain  a 
good  estimate  of  moisture  depletion. 

On  the  cotton  plot,  three  sets  of  seven  tubes  each  were 
placed  at  the  upper,  middle,  and  lower  ends  of  the  440- foot  furrow 
runs.   The  tubes  were  placed  diagonally,  crossing  the  rows,  such 
that  the  tubes  were  located  in  the  plant  row,  and  in  the  furrow 
bottoms  and  furrow  shoulders.  The  number  of  tubes  was  adequate  to 
determine  moisture  change  with  good  precision. 

Cotton  is  not  normally  overirrigated,  which  is  an  advantag 
in  soil  moisture  depletion  studies,  since  soil  moisture  movement  is 
not  as  much  a  problem  in  data  interpretation  as  with  most  other  crop 
Portable  water  meters  were  used  to  measure  the  water  applied  to  the 
cotton.   These  measurements  confirmed  the  seasonal  depletion  record 
obtained  from  the  neutron  probe  measurements. 

Evapo transpirometer  Measurements 

Evapotranspirometers,  sometimes  referred  to  as  lysimeters, 
are  instruments  designed  for  the  measurement  of  evapotranspiration. 
They  can  be  of  various  shapes,  sizes,  and  designs.   Essentially, 


•36- 


they  are  devices  which  enable  the  evaluation  of  the  moisture  recime 
of  a  confined  soil  mass,  of  known  dimensions,  in  which  a  crop  is 
grown.   Moisture  changes  of  the  crop- soil  system  are  determined  by 
periodic  or  continuous  welBhing,  or  by  volumetric  determination  of 
water  displaced,  added,  and/or  removed  from  the  system. 

When  used  for  the  determination  of  field  evapotranspiration. 
it  is  particularly  important  that  the  tanks  be  installed  in  such  a 
manner  that  their  presence  does  not  modify  the  environment  of  the 
measured  crop.   Although  this  technique  appears  to  be  an  excellent 
method  for  precise  measurement  of  crop  water  use,  certain  factors, 
such  as  the  artificial  restriction  of  crop  rooting  and  possible 
modification  of  soil  heat  transfer,  have  yet  to  be  completely  evalua- 
ted.  Research  on  these  factors  is  presently  being  conducted  by  the 
University  of  California. 

The  use  of  evapot ran spirometers  in  the  field  was  not  com- 
mon in  California  at  the  Initiation  of  this  program,  although  tanks 
had  been  used  in  the  1920's  and  1930' s.  Because  soil  moisture  de- 
pletion studies  are  not  adapted  to  crops  frequently  irrigated  or 
having  high  water  tables,  small  evapotranspirometers  were  installed 
to  provide  a  reliable  measure  of  evapotranspiration  under  those 
conditions . 

Alturas-Dorris  Ranch  Evapotranspirometer  Measurements.   In 
1956,  two  small  evapotranspirometers  were  Installed  near  Alturas  to 
measure  evapotranspiration  from  high  water  table  pasture.   The  plot 
site  was  In  an  irrigated  meadow  pasture  containing  high  moisture 
favoring  grasses,  legumes,  and  broad-leafed  plants  found  in  improved 


■37- 


irrigated  pasture  mixes  and  in  native  mountain  meadows.   The  pas- 
ture was  grazed  nearly  continuously  by  cattle,  and  was  usually  short 
but  fully  covered  the  ground.   Typical  percentages  of  green  growin-: 
leaf  surfaces  were  as  follows:   In  April,  40  percent,  increasing  to 
100  percent  by  the  end  of  the  month;  May  through  September,  100  per- 
cent; October,  100  percent,  decreasing  to  50  percent  by  the  end  of 
the  month.   Cover  of  green  foliage  varies  between  zero  and  40  per- 
cent during  the  winter,  depending  to  a  large  extent  upon  the  severit 
of  the  winter.   In  milder  v/inters,  some  green  live  shoots  survive, 
while  in  severe  winters  the  foliage  is  completely  Inactive,  and  the 
green  color  is  gone. 

The  evapotranspirometer  site  v;a5  enclosed  by  a  barbed- 
wire  fence  forming  a  25-  by  75- foot  rectangle.   Inside  the  fenced 
area  the  grass  was  mowed  several  times  during  the  season  to  main- 
tain approximately  a  5-inch  height.   Two  cylind-^ical  steel  evapo- 
transplrometers,  36  Inches  in  diameter  and  30  Inches  deep,  were 
installed  in  the  soil  within  a  fenced  area,  one  at  each  end.   Also, 
inside  the  plot  were  a  hygrothermograph  and  evaporation  pan,  at- 
mometers,  phyheliometer,  and  a  precipitation  gage. 

V/ater  was  supplied  to  the  evapotranspirometers  by  means 
of  a  steady,  small  flow,  at  a  rate  calculated  to  exceed  evapotran- 
spiratlon.   It  took  approximately  one  week  to  utilize  the  water  from 
a  cylindrical  supply  tank  5  feet  deep  and  l8  Inches  in  diameter.   A 
discharge  tube  was  attached  to  the  evapotranspirometer  6  inches  be- 
low the  ground  surface,  and  the  excess  water  not  consumed  in  the 
transpirometer  spilled  into  a  buried  sump  tank,  where  it  was  measurei 

The  numerous  mechanical  problems  encountered  during  the 
first  2.5  years  rendered  the  collected  data  of  questionable  validity 


-38- 


Therefore,  these  data  have  not  been  used  for  this  report.   The 
data  collected  In  19[?9  and  i960  are  considered  representative  of 
evapotransplration  from  hlp;h  water  table  meadov;  pasture,  and  are 
reported  herein. 

Coleville  Evapotransplrometer  Measurements.   In  1957, 
data  on  high  water  table  meadow  pasture  were  collected  from  an 
evapotransplrometer  tank  near  Coleville  in  the  Lassen-Alpine  area. 
The  measurement  site  was  located  at  the  eastern  ed^e  of  the  State, 
at  a  latitude  of  about  38°  30",  at  an  elevation  of  5,100  feet.   The 
site  was  similar  to  the  Alturas-Dorris  Ranch  site  in  vegetation  and 
in  Irrigation  methods.   The  field  was  subject  to  long  irrigations 
by  wild  flooding.   The  v;ater  level  at  this  site  varied  from  O-16 
inches  below  the  ground  surface,  and  was  usually  about  8  inches 
below  the  ground  surface.   A  cylindrical  steel  evapotransplrometer, 
36  inches  in  diameter  by  3  feet  deep,  was  installed.   Water  was 
supplied  to  the  evapotransplrometer  from  a  supply  tank  floated  on 
the  water  table  surrounding  the  evapotransplrometer.   With  this 
system  the  water  table  inside  the  evapotransplrometer  was  kept  at 
essentially  the  same  level  as  that  in  the  field.   Moisture  utilised 
by  the  plants  was  constantly  replaced  from  the  supply  tank.   The 
level  of  the  supply  tank  v;as  recorded  on  a  Stevens  v/ater  stage  re- 
corder.  The  field  water  table  level  was  also  measured  on  a  separate 
recorder.   By  integration  of  the  two  charts,  the  rate  of  evapotranspl- 
ration was  determined* 

The  topography  in  the  area  is  smooth,  with  a  2  percent 
northerly  slope.   Data  were  collected  at  this  site  for  one  season 
in  connection  with  an  investigation  of  water  use  in  watersheds  in 
the  eastern  Sierra  Nevada. 

-39- 


Figure  5  shows  diagraminetlcally  the  functioning  of  the 
Alturas-Dorris  Ranch  and  Coleville  evapotranspirometers . 

Davis  Evapotranspirometer  Measurements.  In  1958,  three 
sinall  evapotransplrometers  2  feet  In  diameter  v;ere  installed  at 
Davis  in  cooperation  v/ith  the  Department  of  Irrigation  of  the  Uni- 
versity of  California.  The  purpose  v;as  to  determine  how  well  thej 
small  tanks  would  compare  v;ith  a  large  20-foot  diameter  tank,  whic 
was  installed  "by  the  university  in  1958.  Over  a  10-month  period,, 
the  mean  evapotransplration  from  the  2- foot  evapotransplrometers 
differed  less  than  5  percent  from  the  20- foot  evapotranspiromete: 
One  reason  for  this  favorable  comparison  is  that  both  kinds  of 
tanks  were  located  in  the  same  field  environment  having  a  con- 
tinuous, uniform  crop  height  and  cover  in  and  around  the  tanks. 
The  data  from  the  2-foot  evapotransplrometers  are  presented  in  thi 
report. 

Evapotransplration  Data  Summary 
Summaries  of  evapotransplration  for  measured  and  esti- 
mated periods  are  tabulated  in  Table  5,  v/lth  corresponding  measure 
ments  of  pan  and  atmometer  evaporation.   Evapotransplration  for 
missing  periods  v;as  usually  estimated  as  the  pix)duct  of  approprlat 
pan  or  atmometer  coefficients,  and  pan  or  atmometer  evaporation 
data  collected  during  these  periods,  plus  calculated  increments  fc 
surface  evaporation  follov/ing  irrigation.   Monthly  evapotranspirat.d 
totals  have  been  computed  and  are  also  presented  in  Table  5-   A 
detailed  tabulation  of  evapotransplration  and  related  data  are  pre- 
sented in  Tables  A-6  and  A-7  of  Appendix  A,  for  the  approximatelv 
v/eekly  measurement  schedule.   Variability  of  soil  moisture  values 

-40- 


pDropping  Orifice  Clock 
11  III Recorder 


High  Water  Table 
Meadow  Grassland 


Recorder 


NOTE-  Intake  and  outflow  columns 
approximate  2"  holes  inside 
tank 


EVAPOTRANSPIROMETER 
36"CIRCULAR  TANK 


OUTFLOW 
TANK 


ALTURAS- DORRIS  RANCH  EVAPOTRANSPIROMETER 


High  Water  Table 
Meadow    Grassland 

1  I  I  I  I  /  /  I  I  I  I  I  i  I  I  11/  /L 


lllllll  I  I II 


Water  Table 


COLEVILLE    EVAPOTRANSPIROMETER 


Figure  5,  EVAPOTRANSPIROMETER    DESIGN 


TABLE  5 


SUMMARY  OP  MEASUREMENTS  OF  EVAPOTRANSPIRATION 
AND  RELATED  DATA 


Year  :  Month 


Evapotransplratlon 


Pan  eT&poratlon 


Meas-  :  Cstl-  :  Accum.  :Monthly  :   Eaoh  lAcoum.  :  Monthly 
ured  :  mated  :  totals  :  est.    t  period  :total8  :  est. 


Atmometer  avaporat 


Eaoh  :Aooum.  :Mon 
period  : totals  :  e» 


Saeramento  River  Mountain  Valley 
Pasture   -  Alturas   -  Dorrls  Ranoh 


Maj 

June 

July 
Aug. 
Sept. 

Oct. 


i960  Apr. 
May 
Jun* 


July 
Aug. 


Sept. 
Oct. 


U/7  -  1+/30 
3/31-  V30 

V30-  5/31 

5/31-  6/30 

6/2  -  6/30 

6/30-  7/31 
7/31-  8/31 
8/31-  9/30 

8/31-  9/22 
9/30-11/2 


V7 

6/2 


V8  -  5/1 
5/1  -  5/31 
5/31-  6/7 
6/7  -  6/14 
6/14-  6/21 
6/21-  6/28 
5/31-  6/30 
6/28-  8/1 
6/30-  7/31 
8/1  -  8/31 
7/31-  8/31 
8/31-  9/30 
9/30-10/31 
9/30-10/3 
10/31-11/21 
11/21-12/1 
10/31-11/30 

12/1  -12/31 

11/30-12/31 


4.03 

5.99 
8.95 
(8.33) 

10.45 
9.04 

4.90 

(3.78) 

3.02 


.11/2    46.38 

.  9/22  31.60 


2.33 
4.61 


10.33 
8.99 

6.01 

3.56 
(0.47) 

0.17 
0.75 


1.96 


1.96 


4.03 

5.15 

'♦.35 

'*.35 

5.54 

10.02 

5.99 

6.09 

10.44 

6.09 

18.97 

8.95 

7.94 

18.38 

7.9'+ 

510 

510    5 

29.42 

10.45 

9.83 

28.21 

9.83 

645 

1,155    6 

38.46 

9.04 

8.65 

36.86 

8.65 

547 

1,702    5 

43.36 

4.90 

5.U1 

42.27 

5M 

315 

2,017 

46.38 

2.85 

3.80 
46.07 

46.07 

3.59 

2,017 

2.33 

2.99 

3.50 

3.50 

'+.53 

6.94 

4.78 

5.71 

9.21 

5.90 

8.90 

1.96 

10.31 

1.73 

12.90 

129 

129 

12.27 

1.96 

116 

13.89 

6.56 

1.98 

16.84 

8.00 

140 

385 

24.22 

9.6I 

9.45 

26.29 

8.82 

608 

993 

33.21 

9.31 

8.02 

34.31 

8.31 

48o 

1,473 

39.22 

6.01 

5.91 

40.22 

5.91 

421 

1,894    u 

42.78 

3.56 

3.81 

44.03 

3.81 

43 

1,934 

43.49 

0.76 

43.66 

0.51 

0.38 

1*5.17 

0.76 

44,4l 

0.78 

0.62 

^5.79 

0.65 

4/6 
6/7 


•12/31 
•10/3 


39.78 
28.83 


1,821 


-42- 


TABLE  3  (oontlnuad) 


SUMMARY  OF  MEASUREMENTS  OF  EVAFOTRANSHIRATION 
AND  REUTED  DATA  (Continued) 


. 

;          ; 

Evapot  ranspl rat 1  on 

Pan  evapo 

ration   :   Atraometer  evaporation 

Meas- 

Estl- : 

Acoua. 

: Monthly 

Eaoh 

:Acoum. 

: Monthly  :   Each  :Accum.  : Monthly 

Y.ar  : Month 

:  Period    : 

ured 

■lated  : 

total. 

:  est. 

period 

: totals 

:  est.   :  period  : totals  :  est. 

Saeramento 

Uver  Basin  Valley  Floo 

Pasture  -  Davis  Caopball 

1959  Jan. 

12/31-  2/2 
12/31-  1/31 

I.U2 

1.42 

1.15 

2.03 

2.03 

1.65 

Fab. 

2/2  -  2/27 
1/31-  2/28 

2.27 

3.69 

2.56 

2.18 

4.21 

2.65 

Mar. 

2/27-  Vi 

?/28-  3/31 

4.1+5 

8.14 

4.31 

6.57 

10.78 

6.32 

Apr. 

Vi  -  '+/16 

2.51 

10.65 

4.26 

15.04 

Vl6-  4/30 

1.82 

12.47 

2.23 

17.27 

3/31-  V30 

4.45 

7.55 

May 

U/30-  5/14 

2.87 

15.34 

4.34 

21.61 

5/14-  5/21 

1.56 

16.90 

2.56 

24.17 

5/21-  5/28 

1.06 

17.96 

1.91 

26.08 

V30-  5/31 

6.05 

9.75 

June 

5/28-  6/8 

2.19 

20.15 

3.67 

29.75 

6/8  -  6/15 

1.67 

21.82 

2.23 

31.98 

6/15-  6/30 

4.08 

25.90 

5.72 

37.70 

5/31-  6/30 

7.38 

11.02 

July 

6/30-  7/29 
6/30-  7/31 

8.11 

34.01 

8.74 

10.74 

48.44 

11.15 

Au«. 

7/29-  8/31 
7/31-  8/31 

7.65 

41.66 

7.02 

9.88 

58.32 

9.13 

S.pt. 

8/31-  9/3 

0.76 

42.42 

0.59 

58.91 

9/3  -10/2 

5.63 

48.05 

8.23 

67.14 

8/31-  9/30 

5.97 

8.14 

Oot. 

10/2  -11/2 
9/30-10/31 

4.26 

52.31 

4.56 

6.66 

73.80 

7.11 

Not. 

11/2  -12/5 
10/31-11/30 

2.12 

54.43 

1.92 

4.19 

77.99 

3.53 

Dae. 

12/5  -12/31 

11/30-12/31 

0.88 

55.31 

(1.25) 

1.68 

79.67 

2.65 

TOTALS   "/31/58-12/3a/55 

44.50 

64.90 

i960  Jan. 

12/31-  1/30 
12/31-  1/31 

0.84 

0.84 

0.88 

1.48 

1.48 

1.63 

Feb. 

1/30-  2/26 
1/31-  2/29 

1.53 

2.37 

1.78 

2.86 

4.36 

3.08 

■^3- 


TABLE  5  (eontinued) 


SUMMARY  OF  MEASUREMENTS  OF  EVAPOTRANSPIRATION 
AND  REUTED  DATA  (Continued) 


TOTALS         5/27-  9/23   28.52 
6/10-  9/23   25.50 

Saoramento  River  Basin  Mountain  Valleys 
Alfalfa  -  Fall  River  Mills  -  Plot  AA 

1959     Mar.            3/17-  U/8  1.92                         1.92 

3/17-  3/31 

Apr.            4/8  -  '4/23  3.86                           5.78 

V23-  V30  1.34         7.12 

3/31.  I+/30 

May              V30-  5/6  1.33          8.45 

5/6  -  5/28  U.2I+                    12.69 

'+/3"-  5/31 

Juno           5/28-  7/2  8.48                       21.17 

5/31-  6/30 

July     7/2  -  7/6  1.27   22.44 

7/6  -  7/27  6.41         28.85 

7/27-  7/31  0.95   29.80 
6/30-  7/31 


31.7'+ 


1,977 


6.76 
6.86 


9.o4 


715 


715 


86  801 
463  1»264 
70   1,33'* 


. 

Evapotransplratlon 

Pan  evapo 

ration 

Atoiomtter  evaporatH 

Meas- 

Esti- 

Acoum. 

: Monthly 

Eaoh 

:Accua. 

: Monthly 

Each 

: Ac cum. 

:Moir 

Year  : Month 

:  Period    : 

ured 

mated 

totals 

:  est. 

period 

: totals 

:  est. 

period 

: totals 

:  ar 

i960  Mar. 

2/26-  3/31 
2/29-  3/31 

3.35 

5.72 

3.06 

4.21 

8.57 

3.86 

Apr. 

3/31-  V29 
4/18-  4/29 
3/31-  4/30 

4.85 
(1.73) 

10.57 

5.27 

5.65 

14.22 

6.11 

138 

138 

May 

4/29-  6/1 
4/30-  5/31 

7.50 

18.07 

7.10 

9.20 

23.42 

8.74 

570 

708 

June 

6/1  -  7/1 
5/31-  6/30 

5.87 

23.94 

5.87 

12.02 

35.44 

12.02 

607 

1,315 

July 

7/1  -  7/19 

4.25 

28.19 

6.44 

41.88 

390 

1,705 

TOTALS 

12/31-  TM 

4/18-  7/19 

28.19 
19.35 

41.88 

1,705 

Lassen  -  Alpine  Mountain  Valleys 

Pasture  -  Colevlll.  -  2E 

1957  May 

5/27-  6/3 

1.34 

1.34 

1.38 

1.38 

June 

6/3  -  6/30 
5/31-  6/30 
6/10-  6/30 

6.91 
(5.23) 

8.25 

7.53 

7.31 

8.69 

7.95 

434 

434 

July 

6/30-  8/1 

9.12 

17.37 

9.12 

9.33 

18.02 

9.33 

601 

1,035 

6c 

Aug. 

8/1  -  8/31 

7.76 

25.13 

7.76 

9.09 

27.11 

9.09 

583 

1,618 

5f 

Sept. 

8/31-  9/23 

3.39 

26.52 

4.63 

31. 7H 

359 

1,977 

-44- 


TABLE  5  (oontlnued) 

SUMMARY  OP  MEASUREMENTS  OF  EVAPOTRANSHIRATION 
AND  RELATED  DATA  (Continued) 


t          : 
:           : 

:vapotran«piratio 

n 

Pan  evaporation 

I  Atraometer  evaporation 

Meas- 

Esti- 

Aooum. 

: Monthly 

Eaoh 

:AceuB. 

: Monthly 

:  Eaoh 

:Acoum. 

: Monthly 

Y«ar  : Month 

:  Period    i 

ured 

mated 

totals 

:  est. 

period 

I  totals 

:  est. 

: period 

: totals 

:  est. 

1?5>  *"«• 

7/31-  8/3 
8/3  -  8/14 
8/1I4.  8/31 
7/31-  8/31 

1.81 

0.86 
3.58 

30.66 
32.47 
36.05 

6.25 

59 
232 

355 

1,393 
1.625 
1,980 

646 

S.pt. 

8/31-  9/3 
9/3  -  9/15 
9/15-  9/30 
8/31-  9/30 

3.'+9 

0.92 
2.51 

36.97 
40.46 
42.97 

6.92 

65 
207 
209 

2,045 

2,252 

2,461 

481 

TOTALS 

3/17-  9/30 
5/28.  9/30 

30.21 
20.19 

1,617 

Sacramento  River  Basin  Mountain  Valleys 

Alfalfa  -  Pall  River  Mills 

-  Plot 

AA 

i960  Mar. 

3/10-  VI9 
3/10-  3/31 

3.71 

3.71 

1.90 

5.91 

5.91 

3.80 

Apr. 

V19-  5/11 
3/31-  H/30 

2.39 

6.10 

2.83 

3.82 

9.73 

4.65 

May 

5/11-  5/19 
5/19-  6/3 
4/30.  5/31 

3.31 

2.37 

8.47 
11.78 

6.42 

2.25 
2.91 

11.98 
14.89 

6.67 

June 

6/3  -  6/24 
6/10-  6/24 

3.29 
(1.21) 

15.07 

6.20 

21.09 

275 

6/24-  6/30 

1.28 

16.35 

1.64 

22.73 

121 

396 

5/31-  6/30 

5.57 

8.69 

588 

July 

6/30-  7/?5 

7.85 

24.20 

8.61 

31.3'+ 

492 

888 

7/25-  8/1 

0.92 

25.12 

2.09 

33.43 

114 

1,002 

6/30-  7/31 

8.40 

10.40 

584 

Aug. 

8/1  -  8/5 

1.06 

26.18 

1.33 

34.76 

80 

1,082 

8/5  -  8/26 

6.30 

32.48 

6.41 

41.17 

375 

1,457 

8/26-  9/1 

1.52 

34.00 

1.46 

42.63 

97 

1,554 

7/31-  8/31 

8.88 

9.20 

550 

Sept. 

9/1  -  9/28 
8/31-  9/30 

4.55 

38.55 

5.10 

5.41 

48.04 

5.85 

425 

1,979 

452 

Oot. 

9/28-11/8 
9/30-10/31 

4.82 

'♦3.37 

3.80 

5.05 

53.09 

3.97 

3/10-11/8   36.22 
6/10-  9/28   19.91 


44.32 


1,567 


-45- 


TABLE  5   (oontinued) 

SUMMARY  OF 

MEASUREMENTS  OF  EVAPOTRANSPIRATION 

AND 

RELATED 

DATA   (Continued) 

: 

Evapotranspirati 

on 

^n  evaporation 

Atmometer  evaporat!  1 

: 

Meas- 

Eati- 

:    Accuo. 

: Monthly 

Each 

:AccuB. 

: Monthly 

Each 

:  Acoura. 

:Mon1 

Year  : Month 

I    Period 

ured 

:    mated 

:    totals 

:    est. 

period 

2  totals 

:    est. 

:   period 

« totals 

:    edi 

Tulare  Lake 

Basin  Valley  Floor 

Alfalfa  -  Arvln  -   Plot  CC 

1959     Mar. 

3/13-  3/27 

1.07 

1.07 

2.28 

2.28 

186 

186 

3/27-  V3 

1.06 

2.13 

1.35 

3.63 

106 

292 

-  3/31 

4.53 

3: 

Apr. 

V3  -  W21 

2.54 

U.67 

4.16 

7.79 

293 

585 

V21-  V28 

1.16 

5.83 

1.77 

9.56 

100 

685 

3/31-  4/30 

4.49 

7.00 

4; 

May 

V28.  ^/Ik 

2.3U 

8.17 

3.82 

13.38 

244 

929 

5/14-  5/25 

1.31 

9.48 

2.96 

16.34 

172 

1,101 

5/25-  6/1 

I.3U 

10.82 

2.58 

18.92 

123 

1,224 

V30-  5/31 

4.54 

8.69 

4: 

June 

6/1  -  6/9 

2.06 

12.88 

2.45 

21.37 

148 

1,372 

6/9  -  6/15 

0.93 

13.81 

1.81 

23.18 

112 

1,484 

6/15-  6/22 

1.00 

11+.81 

2.13 

25.31 

142 

1,626 

6/22-  6/29 

1.45 

16.26 

2.22 

132 

1,758 

5/31-  6/30 

5.80 

9.06 

5: 

July 

6/29-  7/3 

1.09 

17.35 

1.58 

29.11 

88 

1,846 

7/3  -  7/8 

0.90 

18.25 

1.54 

30.65 

102 

1,948 

7/8  -  7/17 

1.20 

19.45 

2.71 

33.36 

170 

2,118 

7/17-  7/22 

1.10 

20.55 

1.37 

34.73 

84 

2,202 

7/22.  7/29 

1.70 

22.25 

2.30 

37.03 

126 

2,328 

6/30-  7/31 

6.34 

9.95 

51 

Aug. 

7/29-  8/8 

2.38 

24.63 

2.83 

39.86 

168 

2,496 

8/8  -  8/13 

0.39 

25.02 

1.67 

41.53 

102 

2,598 

8/13-  8/27 

2.56 

27.58 

3.50 

45.03 

220 

2,8l8 

7/31-  8/31 

6.07 

8.67 

5' 

Sept. 

8/27-  9/15 

3.86 

31.44 

3.96 

48.99 

312 

3,130 

9/15-  9/22 

1.30 

32.74 

1.40 

50.39 

106 

3,236 

9/22-10/2 

I.7U 

34,48 

2.11 

52.50 

158 

3,394 

8/31-  9/30 

5.27 

5.93 

4; 

Oct. 

10/2  -10/9 

0.90 

35.38 

1.04 

53.54 

96 

3,490 

10/9  -10/21 

1.29 

36.67 

1.72 

55.26 

172 

3,662 

10/21-11/3 

i.oU 

37.71 

1.65 

56.91 

135 

3,797 

9/30-10/31 

3.22 

4.49 

4; 

Nov. 

11/3  -  12/2 
10/31-11/30 

2.52 

40.23 

2.76 

2.49 

59.40 

2.69 

Deo. 

12/2  -  1/5 
11/30-12/31 

2.01 

42.24 

1.87 

1.76 

61.16 

1.68 

TOTALS 

3/13-  1/5 
3/13-10/21 

23.78 
19.25 

- 

34.28 

2,069 

-46- 

TABLE  5  (oontlnuvd) 


SUMMARY    OF  MEASUhEMENTS  OP   EVAPOTRANSPIRATION 
AND  REUTED  DATA    (Contlnuad) 


: 

= 

Evapotraneplratlor 

^an  evaporation 

Atmooeter   evaporation 

Meas- 

:   Estl- 

Aoouii.    : 

Monthly 

Eaoh 

:Accuiii. 

: Monthly 

Eaoh 

:Accua. 

: Monthly 

Y>ar 

■.Month 

:    Perl 

od 

ured 

:   mated 

totale  : 

est. 

period 

: totals 

:    est. 

period 

:  totals 

:    est. 

i960 

Mv 

5/12- 

5/31 

2.98 

2.98 

5.58 

5.58 

354 

354 

June 

5/31- 

6/2U 

6.69 

9.67 

8.24 

13.82 

541 

895 

6/21*- 

7/1 

1.01 

10.68 

1.93 

15.75 

138 

1,033 

5/31- 

6/30 

7.67 

9.98 

670 

July 

7/1   - 

7/8 

1.44 

12.12 

2.27 

18.02 

151 

1,184 

7/8- 

8/1 

4.56 

16.68 

7.13 

25.15 

488 

1.672 

6/30- 

7/31 

6.00 

9.39 

639 

Aug. 

8/1  - 

8/10 

1.36 

18. OU 

2.53 

27.68 

180 

1,852 

8/10- 

9/1 

5.27 

23.31 

5.56 

33.24 

402 

2,254 

7/31- 

6/31 

6.63 

8.09 

582 

Sept. 

9/1- 

9/16 

1.67 

2U.98 

3.19 

36.43 

251 

2,505 

9/16- 

9/22 

1.58 

26.56 

1.19 

37.62 

98 

2,603 

9/22- 

9/29 

0.72 

27.28 

1.U1+ 

39.06 

108 

2,711 

8/31- 

9/30 

4.10 

6.13 

480 

Oct. 

9/29-10/27 
9/30.10/31 

2.42 

29.70 

2.59 

3.85 

42.91 

4.08 

341 

2,052 

372 

Nov. 

10/27-11/18 

0.87 

30.57 

1.60 

44.51 

179 

3,231 

TOTALS 

5/12-11/18 

18.69 

26.84 

2,000 

Tulare  Lake  Basin  Valley  Floor 
Cotton  -  Arvin  -   Plot  CD 


1959     MbJ 


4/30-  5/8 
5/8  -  5/21 
5/21-  6/3 
4/30-  5/31 
6/3  -  6/16 
6/16-  6/23 
6/23-  6/30 
5/31-  6/30 
6/30-  7/7 
7/7  -  7/15 
7/15-  7/28 
6/30.  7/31 
7/28-  8/4 
8/4  .  8/11 
8/11-  8/18 
8/18-  8/25 
8/25-  9/2 
7/31-  8/31 
9/2  -  9/24 
8/31-  9/30 


2.67 


'4.55 


1.38 


3.85 


0.13 

0.13 

0.45 

1.51+ 

1.99 

3.87 

2.51 

6.38 

9.05 

11.46 

2.79 

14.25 

18.80 

21.11 

1.86 

22.97 

24.35 

1.50 

25.85 

28.07 

1.58 


7.47 


10.67 


7.76 


5.10 


1.68  1.68 
3.88  5.56 
3.76   9.32 

4.19  13.51 

2.16  15.67 

2.11  17.78 

2.43  20.21 

2.56  22.77 

4.02  26.79 

2.23  29.02 

2.15  31.17 

1.91  33.08 

1.63  3**. 71 

2,23  36. 9t 

4.18  41.12 


8.69 


9.06 


117 

117 

217 

334 

210 

544 

252 

796 

153 

949 

126 

1,075 

138 

1,213 

159 

1,372 

236 

1,608 

126 

1,731+ 

131 

1,865 

125 

1,990 

106 

2,096 

146 

2,242 

8.67 


5.93 


338     2,580 


498 


571 


582 


548 


473 


-47- 


TABLE  5   ( continued ) 

SUl^MARY  OF 

MEASUREMENTS   OP  EVAPOTRANSPIRATION 

AND 

RELATED  DATA   (Continued) 

: 

:                         : 
:                        t 

Evapotranspiration                 : 

Pan  evaporation 

Atmometer   evaporatl( 

Meas- 

Esti- 

:  Aoeum. 

: Monthly  : 

Eaoh 

:Acouiii. 

:  Monthly 

Each 

lAccuni. 

:  Monti 

Y«ar  : Month 

:   Period         , 

ured 

mated 

>    totals 

:    est.        : 

period 

: totals 

:    est. 

period 

: totals 

:    est. 

1959    Oct. 

9/2U.IO/I9 

3.66 

35.60 

4.21 

45.33 

372 

2,952 

Defoliated 

10/19-ll/U 

0.36 

35.96 

2.01 

47.34 

163 

3,115 

9/30.10/31 

2.95 

4.49 

Ul 

Not. 

iiA  -12/17 

10/31-11/30 

0.33 

36.29 

0,19 

3.21 

50.55 

2,69 

1 

TOTALS 

V8  -12/17 
5/8  -llA 

25.96 
25.63 

36.61 

2,239 

1 

Tulare  Lake 

Basin  Valley  Floor 

Cotton  -  Arvln  -  Plot  CP 

i960     Mar. 

3/18-  3/23 

0.50 

0.50 

0.85 

0.65 

Planted     U/6 

Apr. 

3/23-  5/6 
3/31-  V30 

1.46 

1.96 

0.83 

8.40 

9.25 

5.82 

634 

634 

43 

Ma^ 

5/6  -  6/9 
V30-  5/31 

0.37 

2.33 

0.31 

10.39 

19.64 

8.72 

666 

1,300 

57 

June 

6/9  -  6/15 

0.84 

3.17 

2.03 

21.67 

131 

1,431 

6/15-  6/20 

1.81 

4.98 

1.82 

23.49 

124 

1,555 

6/20-  6/30 

2.41 

7.39 

3.20 

26.69 

226 

1,781 

5/31-  6/30 

5.31 

9.98 

67 

July 

6/30-  7/7 

1.94 

9.33 

2.13 

28.82 

138 

1,919 

7/7  -  7/15 

2.63 

11.96 

2.54 

31.36 

179 

2,098 

7/15-  7/28 

4.15 

16.11 

3.59 

3't.95 

249 

2,347 

6/30-  7/31 

10.14 

9.39 

63 

Aug. 

7/28-  8/9 

4.25 

20.36 

3,44 

38.39 

234 

2,581 

8/9  -  8/19 

3.28 

23.64 

2.62 

41.01 

198 

2,779 

8/19-  8/23 

1.43 

25.07 

1.34 

'+2.35 

86 

2,865 

8/23-  8/31 

1.29 

26.36 

1.71 

44.06 

127 

2,992 

7/31-  8/31 

8.87 

8.09 

56 

Sept. 

8/31-  9/21 
8/31-  9/30 

■+.35 

3O071 

5.04 

4.61 

48.67 

6.13 

346 

3,338 

4e 

Oct. 

9/21-10/14 

1.79 

32.50 

3.96 

52.63 

318 

3,656 

Defoliated       -IO/I9 

9/30-10/31 

1.03 

4.08 

37 

Nov. 

10/14-11/22 

10/31-11/22 

0.94 

33M 

1.02 

3.^5 

56.08 

1.89 

365 

4,021 

27 

TOTALS 

3/23-11/22 

22.82 

46.09 

3,398 

MR. 


obtained  during  any  one  period  of  depletion  is  expressed  In  Table  A-6 
under  the  heading  "Twice  the  Standard  Error," 

The  effects  of  percent  ground  cover  and,  possibly,  of 
stage  of  crop  maturity  and  available  soil  moisture,  are  illustrated 
in  Plate  3,    which  compares  accumulated  evapotransplration  of  dif- 
ferent crops.   These  measurements  were  made  in  the  Arvln  area  under 
similar  climatic  conditions  and  on  the  same  soil  series.   Differences 
in  percent  ground  cover  d.nd  possibly  crop  maturity  and  available 
soil  moisture  cause  differences  in  slopes  of  the  curves  shown  in 
Plate  3.   Defoliation  caused  the  abrupt  changes  in  evapotransplration 
rates  reflected  in  the  curves  on  cotton  and  plums.  Alfalfa  remains 
green  at  this  location  throughout  the  year,  and  shows  little  seasonal 
slope  changes.   It  is  of  interest  to  note  the  much  higher  July  and 
August  rates  of  evapotransplration  by  cotton,  as  compared  to  alfalfa 
and  plums  in  both  1959  and  1960.  A  complete  explanation  for  this 
cannot  be  presented;  however,  certain  of  the  factors  affecting  evapo- 
transplration are  discussed  in  the  following  chapter. 


-49- 


CHAPTER  IV.   CORRELATION  OF  EVAPOTRANSPIRATION 
DATA  WITH  AGROCLIMATIC  DATA 


To  attempt  concurrently  to  measure  evapotransplration 
of  the  many  species  of  Irrigated  crops  presently  grovm  In  Cali- 
fornia is  impractical  because  of  financial  and  manpower  require- 
ments.  Likewise  impractical  is  the  measurement  of  evapotransplration 
of  a  single  crop  at  more  than  a  few  locations. 

The  most  promising  approach  at  this  time  appears  to  be 
to  determine  the  important  and  measurable  parameters  affecting 
evapotransplration  rates,  and  to  correlate  actual  measurements  of 
evapotransplration  with  those  parameters .   Three  Important  para- 
meters which  appear  independently  to  affect  evapotransplration  are 
climate,  plant  conditions,  including  physiological  factors,  and 
soil  moisture  availability.   Differences  in  the  physical  and  chemical 
properties  of  soils  and  soil  fertility  are  not  considered  to  directly 
affect  evapotransplration,  even  though  they  may  have  indirect  effects, 

This  chapter  discusses  the  relationship  of  each  of  those 
parameters  of  evapotransplration,  and  summarizes  the  analysis  of 
data  collected  through  1960.   In  this  regard,  basic  research  on 
factors  affecting  evapotransplration  is  being  conducted  by  the  Uni- 
versity of  California,  as  an  integral  part  of  the  Vegetative  Water 
Use  Program.   The  Agricultural  Research  Service  is  also  conducting 
basic  research  in  this  field.   The  results  of  these  research  pro- 
grams have  affected,  and  shall  continue  to  Influence,  the  course 
of  these  studies. 


■51- 


Evapotransplratlon  and  CllTnatic  Data 
Climate  In  the  evapoti^ansplratlon  process  can  be  though 
of  as  a  combination  of  evaporative  elements,  such  as  air  tempera- 
ture,  wind,  dryness  of  the  air,  and  solar  radiation.   Other  fac- 
tors of  climate,  such  as  length  of  daylight,  may  be  indirectly 
related  to  evaporation. 

The  energy  sources  for  the  evapotransplratlon  processes 
are  derived  principally  from  direct  solar  radiation  and  advection 
or  exchangeable  heat  from  the  air.   The  evaporative  demand  of  the 
atmosphere  is  largely  a  function  of  those  tv70  elements.   However, 
not  all  of  the  solar  radiation  that  falls  directly  on  the  plant 
or  ground  surface  is  used  in  evapotransplratlon.   A  portion  is  re 
fleeted  back  into  the  atmosphere,  a  portion  Is  utilized  in  heatin 
the  air,  a  portion  is  absorbed  in  heating  the  soil,  and  the  balan 
is  utilized  in  evapotransplratlon  and  plant  growth.   It  is  likev^i; 
probable  that  the  energy  available  from  advection  is  not  all  uti- 
lized, depending  upon  many  factors,  such  as  vapor  pressure  deficis 
and  extent  of  v/lnd  movement.   Under  certain  conditions,  it  has  bei 
demonstrated  that  advective  cooling,  as  well  as  advecting  heating 
may  occur. 

As  the  moisture  content  of  the  air  increases  through 
evapoi'atlon  and/or  transpiration,  the  moisture  gradient  (vapor 
pressure  gradient)  between  an  air  mass  and  an  evaporating  surface 
becomes  less  steep  and  retards  further  moisture  transfer.   Under 
field  conditions,  the  air  mass  near  the  ground  is  far  from  stable 
Air  movements  act  to  mix  moisture- saturated  air  near  the  evaporatn 

-52- 


surface  with  drier  air  from  above.   Wind  speeds  and  surface  rough- 
ness Influence  the  relative  turbulence  of  the  air,  moving  the 
moisture  away  from  the  evaporating  surface  and  bringing  In  drier 
air  to  further  the  evaporation  process.   Thus,  it  is  apparent  that 
the  evaporative  demand  of  the  atmosphere  is  determined  by  the  inter- 
action of  several  climatic  elements. 

Progress  is  being  made  in  determining  the  relationships 
between  the  aforementioned  climatic  factors  to  arrive  at  a  quan- 
titative approach  to  estimating  evapotranspiration. 

Evapotransplratlon  and  Plant  Conditions 
The  term  "evapotransplratlon"  implies  the  sum  of  evapo- 
ration plus  transpiration.   In  the  case  of  plants  that  are  actively 
growing  and  well  supplied  with  moisture,  transpiration  is  related 
and  responsive  to  climatic  conditions.   Evaporation  from  soils,  hov/- 
ever,  is  related  more  closely  to,  and  limited  by,  the  moisture  con- 
tent of  the  exposed  soil  surface  than  to  climatic  conditions.   In 
most  irrigated  areas  in  California,  rain  is  sparse  during  the  growing 
season  and,  except  for  areas  of  high  water  tables,  soil  svirfaces 
soon  dry  through  evaporation  following  irrigation.   As  a  result, 
under  California  irrigation  conditions,  transpiration  is  usually 
the  larger  of  the  two  components  comprising  evapotransplratlon. 

The  primary  plant  parameter  affecting  evapotransplratlon 
rate  appears  to  be  the  percent  of  ground  cover.   This  is  an  im- 
portant consideration  when  determining  evapotranspiration  for  an- 
nual field  crops,  such  as  sugar  beets  and  cotton,  and  for  other 


53- 


crops  having  variable  ground  cover  percentages,  such  as  alfalfa, 
which  is  cut  frequently. 

Crops  having  rapid  growth  rates  and  vigor  tend  to  provld 
greater  ground  cover  more  rapidly  than  a  slow-grov/ing  crop,  even 
of  the  same  species.   Thus,  differences  in  growth  rate  may  affect 
evapotranspiration  rates  through  the  direct  mechanism  of  percent 
of  ground  cover,  although  other  physiological  factors,  such  as 
stage  of  maturity  or  growth,  may  also  affect  evapotranspiration. 

Evapotranspiration  and  Soil  Moisture 
Research  findings  relative  to  the  effect  of  variations 
of  available  soil  moisture  upon  evapotranspiration  and  plant  grov/t 
are  varied. 

The  amount  of  soil  moisture  available  above  the  permaner 
wilting  point  does  not  seem  to  affect  the  evapotranspiration  rate 
of  crops,  according  to  many  research  reports.   Other  research  has 
indicated  that  maximum  growth  rates  are  obtained  only  under  condi- 
tions of  high  moisture  availability,  and  that  growth  rates  and 
yields  are  retarded  as  soil  moisture  availability  decreases.  Thes 
concepts  differ  from  other  research  investigations  which  have  indi 
cated  a  close  relationship  between  evapotranspiration  and  plant 
growth.  These  concepts  are  of  particular  Importance  in  conslderir 
if  evapotranspiration  rates  are  affected  by  lov;  soil  moisture  leves 
vjhlch  appear  to  affect  growth  rates,  such  as  occur  v;hen  irrigatior 
is  deliberately  withheld  from  grapes  and  cotton  to  change  their 
fruiting  characteristics. 

Besides  intentional  withholding  of  irrigation,  there  are 
also  occasions  of  drought  due  to  insufficient  irrigation  water 


__54- 


supplies.   Due  to  the  foregoing  reasons,  crops  are  frequently  sub- 
jected to  drought  for  periods  of  time  varying  from  a  few  hours  up 
to  several  weeks . 

Therefore,  In  the  studies  reported  here,  an  evaluation 
was  made  of  the  effect  of  available  moisture  upon  evapotransplratlon, 

Available  moisture  was  determined  for  the  principal  root 
zone  for  each  crop  from  selected  neutron  probe  soil  moisture  data. 
In  the  case  of  the  alfalfa,  a  perennial  crop,  a  single  zone  from 
0-12  feet  was  used  for  the  entire  study.   For  cotton,  an  annual 
crop,  the  zone  was  increased  from  the  0-1-foot  depth  to  the  0-11- 
foot  depth  as  the  crop  grew  and  the  root  system  developed  and  ex- 
panded. The  results  of  the  evaluations  are  discussed  further  under 
the  sections  on  crop  coefficients. 

Other  Factors  Affecting  Evapotransplratlon 
Soil  fertility  and  other  physical  factors  of  the  soil, 
such  as  texture,  structure,  salinity,  and  even  color,  affect  the 
growth  rate  of  a  crop.   Soil  properties,  such  as  texture,  struc- 
ture, and  salinity  may  also  affect,  to  some  degree,  moisture  move- 
ment and  utilization.  These  factors  have  an  undetermined,  and 
probably  much  lesser,  effect  on  evapotransplratlon  than  drought, 
climate,  and  plant  conditions. 

Determination  of  Coefficients 
Results  of  various  research  projects  have  indicated 
that  the  processes  of  evapotransplratlon  and  evaporation  are  both 
responsive  to  the  same  factors.   As  will  be  discussed  in  ensuing 


-55- 


paragraphs,  a  definite  relationship  exists  between  evapotranspi- 
ration  and  rates  of  evaporation  from  pans  or  atmometers .   This 
relationship  is  considered  fundamental  to  estimating  evapotranspi- 
ratlon  for  other  crops  and  in  other  agricultural  areas  throughout 
the  State. 

The  ratio  of  evapotranspiration  (ET)  to  evaporation  from 
an  evaporation  pan  (Ep)  is  referred  to  as  a  "pan  coefficient" 
(ET/Ep)  ;  in  like  m.anner,  the  ratio  of  evapoti-anspiration  to  net 
atmometer  evaporation^  or  the  difference  of  evaporation  from  a 
black  and  white  Livingston  Spherical  Atmometer  (Eb-w),  is  referred 
to  as  the  "atmometer  coefficient"  (ET/eb-w) . 

Pan  and/or  atmometer  coefficients  for  individual  evapo- 
transpiration measurement  periods  for  the  various  plots  sampled 
are  shown  in  Appendix  A  in  Tables  A-6  and  A-7.   A  casual  examina- 
tion of  these  individual  periods  reveals  v;ide  variations  which 
v/ould  appear  to  discount  the  validity  of  such  comparisons.   How- 
ever, a  more  detailed  analysis  of  the  data  indicates  that  ce^-tain 
relationships  do  exist,  and  upon  such  relationships  tentative 
values  can  be  established.   Certain  variations  of  the  pan  and  at- 
mometer coefficients  from  time  to  time  are  caused  by  plants  re- 
sponding differently  to  evaporation  influences  than  do  pans  and 
atmometers.   Likewise,  variations  in  the  coefficients  were  due 
also  to  individual  differences  in  the  response  of  atmometers  or 
pans  to  these  climatic  influences. 

Analysis  of  data  for  each  individual  crop  and  the  con- 
clusions drawn  therefrom  are  discussed  in  the  follo\\fing  paragraphs 


56- 


Grass  and  Pasture  CoelTlolents 

Pan  and  atmometer  coerflclents  have  been  determined 
using  data  from  gx^ass  and  grass-pasture  evapotransplrometer  tanks 
located  In  the  Sacramento  River  Basin  mountain  valleys,  in  the 
Lassen-Alpine  mountain  area,  and  in  the  Sacramento  Valley  floor 
(Alturas,  Coleville,  and  Davis). 

Graphs  of  coefficients  and  percent  of  ground  cover  for 
pasture  and  grass,  plotted  against  time,  are  presented  in  Figures  A 
through  E  of  Plate  ^,  entitled  "Variation  of  Pan  and  Atmometer  Co- 
efficients for  Individual  Periods  of  Measurements."   Percent  of 
ground  cover  is  relatively  constant  for  those  crops,  and  wide  varia- 
tions of  the  coefficients  occur  less  than  with  alfalfa  and  cotton. 
During  the  growing  period,  the  grass  was  at  nearly  100  percent 
grovind  cover  in  all  of  the  evapotransplrometer  tanks,  as  mowings 
did  not  clip  the  foliage  short  enough  to  cause  large  reductions 
in  ground  cover.   While  the  ground  was  alv;ays  sod- covered,  the 
colder  climate  at  the  mountain  sites  caused  dormancy  to  some  de- 
gree during  late  fall,  V7inter,  and  early  spring.   Approximate 
ground  cover  percentages  indicated  on  Figures  A,  B,  and  E  of 
Plate  4  are  for  the  green  and  actively  growing  fraction  of  the 
foliage.   At  Davis,  the  climate  is  not  cold  enough  to  force  the 
grass  completely  into  winter  dormancy.   Occasionally  at  the  Davis 
site,  however,  small  areas  of  ground  surface  were  exposed  through- 
out the  year,  as  indicated  on  Plate  4,  Figures  C  and  D. 

High  water  table  conditions,  typical  of  the  predominant 
irrigation  practice  in  the  mountain  valleys,  were  maintained  in 
the  Alturas  and  Coleville  tanks.   There  was,  therefore,  no 


■37- 


moisture  shortage  at  the::e  sites.   The  cvapotranspirometer  tanks 
and  ryegrass  rield  at  Davis  were  frequently  Irrigated,  and  it  is 
probable  thiat  soil  moisture  was  not  limiting  there.   Availability 
of  soil  moisture  Is  assumed  to  have  had  little  effect  on  evapo- 
transpiratlon  rates  and  coefficients  at  any  of  the  three  sites. 

Seasonal  accumulated  evapotransplratlon  plotted  against 
accumulated  pan  evaporation  and,  except  for  Davis,  accumulated 
atmometer  evaporation  are  shown  on  Plate  ^,    entitled  "Comparison 
of  Pan  and  Atmometer  Coefficients  for  Cotton,  Alfalfa,  and  Grass,' 
Figures  E  and  F.   Each  curve  is  for  an  individual  year,  and  has 
separate  zero  lines  for  plotting  evapotransplratlon.   Evaporation 
from  pans  or  atmometers  vras  plotted  using  the  date  of  June  30  as 
the  common  point  on  all  curves.   Coefficients  for  the  period  of 
record  for  both  years  were  consistently  similar  for  Alturas  for 
both  pan  and  atmometer.   The  pan  coefficient  for  the  period  of 
record  at  Davis  was  likewise  similar. 

Coefficients  from  three  seasons  of  record  in  the  mounta: 
areas,  combining  Alturas  and  Coleville,  are  compared  with  coeffic:i 
from  Davis  in  Table  6.   Coefficients  are  shown  for  both  the  growii 
seasons  assumed  in  Bulletin  No.  2  and  for  the  longer  period  for 
v;hlch  data  were  obtained.   The  reason  for  the  differences  between 
the  valley  and  mountain  coefficients  has  not  been  ascertained. 

Alfalfa  Coefficients 

Pan  and  atmometer  coefficients  have  been  determined  froi 
an  alfalfa  plot  located  near  Pittville  in  the  Sacramento  River  Ba;t! 
mountain  valleys,  and  from  an  alfalfa  plot  near  Arvin  in  the  Tula:! 
Lake  Basin  Valley  floor  at  the  southern  end  of  the  Central  Valley 


•58- 


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■59- 


One  of  the  most  notable  details  of  the  alfalfa  coef- 
ficients determined  from  both  areas  is  the  variation  associated 
with  percentage  of  ground  cover.   It  is  important  to  point  out 
that  the  method  of  collecting  data  on  percentage  of  ground  cover 
was  subjective^  being  based  upon  personal  judgment,  and  that  esti- 
mates by  individual  observers  differ  by  perhaps  5  to  15  percent. 
There  is,  however,  general  agreement  that  following  mowing  the 
ground  cover  is  usually  reduced  to  5  to  10  percent,  and  that  grour. 
cover  usually  approaches  100  percent  cover  prior  to  mov/ing .   Al- 
though there  are  exceptions  due  to  possible  experimental  error 
and  other  factors,  the  coefficients  are  smaller  when  the  ground 
cover  is  low  following  mowing,  and  become  larger  as  the  ground 
cover  increases.   Plate  4,  Figures  F,  G,  H,  and  I,  illustrate 
these  relationships  between  coefficients  and  percent  of  ground 
cover,  plotted  against  time. 

A  more  direct  comparison  of  pan  and  atmometer  coefficien: 
with  percent  of  ground  cover  is  sho\m  in  Plate  6,  entitled  "Rela- 
tionship Between  Pan  and  Atmometer  Coefficients  for  Alfalfa  and 
Ground  Cover."   Figure  A  shows  atmometer  coefficients,  and  Figure 
shows  pan  coefficients.   The  data  for  both  figures  were  the  same 
utilized  in  Plate  4.   As  indicated  in  Plate  6,  the  Pittville  co- 
efficients appear  to  be  higher  than  the  Arvin  coefficients.   Two 
linear  regression  lines  have  been  fitted  to  the  data.   However, 
it  may  be  that  additional  data  will  indicate  a  somewhat  curvi- 
linear relation.   It  seems  reasonable  to  assume  that  coefficients 
at  100  percent  of  ground  cover  would  not  be  proportionally  higher 
than  coefficients  at  80  percent  of  gi'ound  cover,  which,  for  prac- 
tical purposes,  also  provide  nearly  complete  shade,  except  near 
noonday. 

-60- 


Since  the  soil  at  both  plot  sites  was  deep,  and  alfalfa 
is  a  perennial  crop,  moisture  in  the  0-12-foot  zone  was  used  to 
estimate  available  soil  moisture. 

The  lowest  moistures  occurred  at  the  Pittville  plot, 
where  on  several  occasions  the  available  moisture  was  reduced  to 
less  than  2  inches  in  the  12  feet,  or  less  than  0.2  inch  of  moisture 
per  foot  of  soil,  on  the  average.   When  this  condition  occurred, 
the  upper  portion  of  the  profile  was  usually  relatively  drier  than 
the  deeper  soil.   On  several  of  these  occasions,  crop  growth  at 
the  Pittville  plot  was  slow,  and  considerable  flower  blooms  and 
dark  blue-green  leaf  color  associated  with  moisture  deficiency 
appeared.   As  indicated  on  Figures  F  and  G  of  Plate  4,  low  available 
soil  moisture  may  account  for  some  of  the  smaller  coefficients  noted 
prior  to  mowing.   The  Arvin  plot,  in  contrast,  was  very  well  supplied 
with  soil  moisture.   As  shown  on  Figures  H  and  I,  the  available 
moisture  at  Arvin  ranged  above  1  and  up  to  2  inches  per  foot  during 
the  measurement  periods. 

If  evapotranspa ration  were  reduced  by  low  available  soil 
moisture,  the  pan  and  atmometer  coefficients  would  be  smaller. 
This  does  not  appear  to  be  the  case  for  the  Pittville  plot,  al- 
though several  of  the  coefficients  just  prior  to  mowing  are  smaller 
than  would  be  expected,  considering  the  percent  of  ground  cover. 
Overall,  the  pan  and  atmometer  coefficients  of  the  Pittville  data 
are  as  high,  if  not  higher,  than  the  Arvin  coefficients,  regard- 
less of  the  lower  soil  moistures  at  Pittville. 

Since  coefficients  from  the  Pittville  and  Arvin  plots 
show  monthly  variations  reflecting  mowing  schedules,  farm  practices, 

-61- 


and,  perhaps,  effects  of  plant  (growth  environments.  It  is  deemed  ■, 
best  to  compare  seasonal  rather  than  monthly  coefficients. 

Seasonal  coefficients  have  been  determined  for  periods 
when  evapotranspiration  measurements  were  made  using  data  shown 
in  Table  5^  and  are  summarized  here  as  follows: 

Seasonal  Alfalfa  Coefficients  Determined 
From  Measured  Periods  Only 

Pan  Coefficient       Atmometer  Coefficient 

1939        i960 

0.0125      0.0127 

0.0093      0.0093 

In  order  to  take  into  account  the  possibility  that  the 
sampling  periods  could  be  biased  and  not  representative  of  groundl 
cover  conditions,  and  also  to  include  estimated  evaporation  incre 
ments  following  irrigations,  estimates  of  evapotranspiration  were 
made  for  the  irrigation  periods.   These  estimates  fill  in  the  misii 
records.   Seasonal  coefficients  determined  from  these  data  are  su- 
marized  as  follows: 


Seasonal  Alfalfa  Coefficients, 
Including  Estimated  Data 

Pan  Coefficient       Atmometer  Coefficient 


1959 

i960 

Pittville 

nr 

0.82 

Arvin 

0.69 

0.70 

1959 

i960 

1959 

i960 

Pittville 

nr 

0.82 

0.0123 

0.0125 

Arvin 

0.69 

0.69 

0.0099 

0.0095 

The  close  similarity  between  the  coefficients  determine 
from  the  measured  periods  as  compared  with  the  total  seasonal 
period  of  record,  including  estimated  periods,  indicates  that  the 


-62- 


measured  periods  are  not  biased,  and  the  seasonal  coefficients 
appear  to  be  reasonable.   Curves  of  seasonal  accumulated  evapo- 
transplratlon  versus  evaporation  are  shovm  on  Plate  5,  Figures  C 
and  D.   As  noted  previously,  each  curve  on  Plate  ^j   is  plotted  for 
an  individual  year,  with  separate  zero  lines  for  indication  of 
evapotranspiration.   Evaporation  from  pan  and  atmometers  was  plotted, 
using  the  date  of  June  30  as  the  common  point  on  all  curves. 

The  pan  and  atmometer  coefficients  derived  after  com- 
bining the  two  years  of  record  at  the  Pittville  AA  plot  are  shown 
in  Table  7 ,    and  are  compared  with  coefficients  derived  in  the  same 
manner  at  the  Arvin  CC  plot.   For  purposes  of  comparison,  average 
coefficients  were  determined  not  only  for  the  period  of  record, 
but  also  for  the  growing  season,  as  shown  in  Bulletin  No.  2. 

By  any  method  of  determining  seasonal  coefficients,  the 
Pittville  pan  coefficient  is  approximately  17  percent  higher  than 
the  Ai'-vin  coefficient,  and  the  Pittville  atmometer  coefficient  is 
approximately  27  percent  higher  than  the  Arvin  coefficient.  Whether 
the  difference  is  due  primarily  to  basic  climatic  differences  be- 
tween the  two  areas,  v/hlch  affect  different  plant  and  evaporation 
response,  or  due  to  experimental  error,  is  not  knovm  at  this  time. 

Cotton  Coefficients 


Pan  and  atmometer  coefficients  for  cotton  for  each  period 
of  measurement  during  1939  and  196O  are  shown  in  Figures  J  and  K 
of  Plate  4.   Also  shov/n  are  estimates  of  the  percent  of  ground 
cover,  available  moisture,  and  other  factors  affecting  plant  growth 
and  water  use.  There  is  a  rather  close  agreement  betv/een  the  two  years 


■63- 


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-64- 


in  regard  to  the  general  pattern  of  plant  growth  and  the  relation- 
ships of  the  coefficient  with  the  various  factors  affecting  water 
use.   The  soil  moisture  observations  are  believed  to  be  of  reliable 
quality,  particularly  for  the  i960  data,  where  a  dry  soil  zone  v;as 
maintained  at  depth  below  the  root  zone,  assuring  that  no  deep  per- 
colation of  Irrigation  water  occurred. 

Coefficients  for  individual  period  of  measurements  show 
a  pattern  of  progressive  increase  from  the  lov;  early  season  value 
to  a  peak  in  July,  and 'then  a  progressive  decrease  to  the  year's 
end.   The  differences  in  coefficients  during  the  early  season 
emphasized  the  direct  relationship  between  the  evapotranspiration 
and  percent  of  ground  cover.   The  decreasing  pattern  of  coefficients 
after  July  reflects  the  integrated  effect  of  decreasing  ground 
cover,  physiological  aging  of  the  plants,  and  availability  of 
soil  moisture. 

It  is  of  interest  that,  although  ground  cover  on  these 
plots  reached  80  and  95  percent,  the  maximum  coefficients  were 
reached  at  a  ground  cover  of  about  60  percent.  This  corresponds 
in  time  to  the  boll  setting.   It  is  believed  that  physiological 
factors  may  have  had  an  influence  on  the  transpiration  rate  at 
this  stage  of  plant  development.   Physiological  factors  are  be- 
lieved to  have  caused  similar  effects  in  other  crops.   With  small 
grains,  for  example,  peak  water  use  rates  are  reported  to  occur 
at  the  heading  stage. 

Late-season  use  of  water  by  cotton  is  dependent  to  some 
extent  upon  the  amount  of  moisture  available  prior  to  natural  or 


-65- 


Ind-uced  defoliation.   The  plants  will  generally  use  all  available 
moisture  within  the  root  zone.   The  amount  of  use  is  a  function 
of  the  amount  of  moisture  available.   This  is  to  say  that  the  avail 
ability  of  soil  moisture  is  often  the  limiting  factor  in  the  late- 
season  evapo transpiration.   This  also  may  account^  in  part,  for  the 
August-September  coefficients  being  lower  than  the  July  coefficieni 

Early-  or  late-season  precipitation,  although  a  part  of 
evapotranspiration  and  reflected  in  the  pan  and  atmometer  coeffic- 
ients, is,  quite  often,  not  a  beneficial  source  of  moisture  to  the 
plants.   Early-season  precipitation  is  evaporated  from  the  soil  su: 
face  with  little  gainful  effect  upon  plant  grov/th.   Late-season 
precipitation  is  either  evaporated  from  the  soil  or  vegetative  sup- 
face,  and/or  transpired  by  the  plant  without  contributing  substanti,, 
ly  to  the  plant  cultural  requirements.   Thus,  pan  and  atmometer 
coefficients  for  early  and  late  season  must  be  applied  with  cautic: 
and  only  after  a  thorough  evaluation  of  rainfall  amount,  frequency 
and  pattern,  as  well  as  knowledge  of  the  late- season  availability 
of  soil  moisture. 

Based  on  the  information  summarized  in  Table  5^  monthly 
pan  and  atmometer  coefficients  for  the  two  years  of  record  have 
been  determined,  and  are  shown  on  Figures  A  and  B  on  Plate  b. 
There  is,  in  general,  rather  close  agreement  between  the  monthly 
pan  or  atmometer  coefficients  for  both  1959  and  i960.   There  are 
also  several  indications  that  evapotranspiration  for  cotton  some- 
times exceeds  evaporation  from  pans.   The  July  pan  coefficients 
for  1959  and  i960  were  respectively  I.07  and  I.08,  which  indicates 
that  evapotranspiration  exceeds  evaporation  from  the  free-water 

-66- 


surface.   It  is  believed  that  the  crop  surface  roughness  may, 
thi'ouch  greater  air  niixln,":,  be  one  of  the  influencing  factors. 

Average  monthly  pan  and  atmometer  coefficients  for 
cotton  for  the  tv;o  years  of  record  are  presented  in  Table  8. 
For  purposes  of  comparin;c  v/ith  Bulletin  2  estimates,  an  average 
coefficient  for  tlie  Tulare  Lake  Basin  Valley  Floor  Hydrographic 
Units  was  determined  for  the  grov/in;-  season  used  in  Bulletin  2. 
For  the  period  from  May  through  October,  the  active  growing  season, 
the  average  pan  coefficient  is  0.68,  and  the  atmometer  coefficient 
is  0.0098.   The  monthly  coefficients  for  the  period  from  June 
through  September  are  considered  to  be  primarily  the  effect  of 
climatic  evaporative  demand  and  crop  conditions,  and  are  not  sub- 
ject to  the  influence  of  early-or  late-season  nonbeneflcial  uses. 

Application  of  Coefficients  and  Evaporation 
Data  to  Estimation  of  Evapotranspiration 

Using  the  average  pan  or  atmometer  evaporation  observed 
in  each  area,  as  shown  in  Tables  2  and  3  in  Chapter  II,  and  ap- 
plying the  appropriate  pan  or  atmometer  coefficients  as  described 
jn  Tables  6,  7j  and  8,  estimates  of  monthly  consumptive  use  values 
viere   made  for  several  crops.   These  monthly  estimates  are  summarized 
in  Table  9,    and  are  compared  v;it.j  values  utilized  in  Bulletin  2, 
"Water  Utilization  and  Requirements  in  California,"  published  by 
the  department  in  195b •   To  make  the  comparison  with  Bulletin  2 
values  valid,  the  grov/ing  seasons  used  in  Bulletin  2  were  used 
in  all  calculations  for  Tables  6  through  9-      In  general,  the  esti- 
mates based  upon  the  pan  and  atmometer  coefficients  are  approximately 
equal  to  or  ;;reater  than  the  Bulletin  2  values.   This  is  also  true 
where  measured  values  of  consumptive  use  are  available.  This,  in 


TABLE  8 
PAN  AND  ATOOMETER  COEFFICIENTS  FOR  COTTON 


Pan  Coefficients 


Month 


Tulare  Lake  Basin  Valley  Floor 
(Arvin  (CD)   1959 
Arvin  (CF)   19 60) 


No.  Days 

of  Record       : 


ET/Ep» 


Atmoraeter  Coefficients 


Tulare  Lake  Basin  Valley  Flo" 
(Arvin  (CD)  1959 
Arvin  (CF)   I96O) 


No.  Days 
of  Record 


ET/Eb-w» 


January 

■— 

~ 

February 

- 

— 

March 

a 

0.75 

April 

30 

O.lli 

May 

62 

0.11 

June 

60 

0.67 

July 

62 

1.08 

August 

62 

0.99 

September 

60 

0.8U 

October 

62 

0.U6 

November 

60 

0.26 

December 

62 

0.15 

30 

0.0019 

62 

0.0018 

60 

0.0103 

62 

0.0170 

62 

0.011i7 

60 

0.0106 

62 

0.0051 

60 

0.0023 

Average 
Coefficient 
for  Growing 
Seasonl/ 

Period  of 
Record 

398 

0.68a/ 

398 

0.0098*/ 

2/  For  growing  season  periods  used  in  Bulletin  No.  2,   Tulare  Lake  Basin  Valley  Fm 

hydrographic  units. 
•/  April-October 
♦    ET/Ep  -  Pan  Coefficient,  ET/Eb-w  -  Atmometer  Coefficient 


-68- 


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Itself,  does  not  prove  that  the  estimates  based  upon  pan  and  at- 
mometer  coefficients  are  more  accurate.   Additional  supporting 
data  shall  be  required  to  confirm  this  possibility. 

Examination  of  the  data  in  Table  9  indicates  that  esti- 
mates of  consumptive  use  can  be  made  with  equal  confidence,  on  th( 
basis  of  either  pan  or  atmometer  data.   It  should  be  emphasized 
also  that  the  consumptive  use  values  must  be  determined  for  the 
actual  period  of  active  plant  grov/th.   The  actual  grov^ing  season 
for  most  crops  in  the  various  areas  of  the  State  still  remains  to 
be  determined.   Furthermore,  a  careful  analysis  of  precipitation 
pattern,  frequency,  and  amounts  must  be  made  for  both  growing  and 
nongrov^ing  seasons,  to  determine  the  effectiveness  of  this  moistui 
source  toward  meeting  the  water  demand  of  the  various  crops. 


-70- 


CHAPTER  V.   SUMMARY,  CONCLUSIONS,  AND  RECOMMENDATIONS 

This  chapter  presents  a  concise  summary  of  the 
ve,?;etatlve  water  use  studies,  the  conclusions  drawn  therefrom, 
and  rocotninendatlons  v/lth  regard  to  the  future  lines  of  study. 

Summary 

Precise  Jcnowi  edj-e  of  the  total  seasonal  as  well  as 
the  distribution  pattern  of  water  use  throughout  the  year  is 
basic  to  the  plannlnc>  desi;;n,  and  operation  of  comprehensive 
water  development  projects.   In  developing  this  essential 
knov/ledge,  the  department  has  been  engaged  in  studies  directed 
tov;ai'd  determination  of  evapotranspiration.   During  the  period 
from  193^1  to  i960,  these  studies  v/ere  limited  to  certain  geo- 
graphic regions  of  northern  and  central  California. 

Accurate  measurement  of  evapotranspiration  is  so 
complex  and  costly  that  practical  considerations  limit  collec- 
tion of  these  data  to  relatively  fev7  locations.   Recent  re- 
search work  by  various  groups  throughout  the  world  has  pointed 
out  certain  fundamental  relationships  between  the  evapotranspira- 
tion process  and  climatic  factors.   Transpiring  crops  respond 
to  the  same  energy  sources  as  evaporation  devices.   The  response 
of  crops,  however,  is  modified  by  physical  and  physiological 
characte':'istlcs.   Under  any  given  climatic  condition,  factors 
such  as  availability  of  soil  moisture,  percent  of  vegetative 
gi'-ound  cover,  and  physiological  development  control  the  rate 
of  ovapoti'anspiratlon. 


-71- 


The  approach  taken  in  these  studies  has  been  to 
study  at  a  few  locations  the  relationship  between  measured 
evapotranspiration,  under  specified  crop  conditions,  and  certain 
climatic  indices.   Concurrently  with  the  measurement  and  corre- 
lation of  evapotranspiration  and  climatic  factors,  a  netv;ork  of 
asroclimatic  stations  was  established  and  observed  throughout 
the  several  major  inland  agricultural  areas  in  northern  and  central 
California   Having  determined  evaporation  at  these  stations,  esti- 
mates of  evapotranspiration  can  be  extrapolated  into  these  areas 
by  using  the  relationship  between  evapotranspiration  and  evapora- 
tion data  measured  at  the  key  evapotranspiration  stations. 

In  the  early  years  of  these  studies,  available  knowl- 
edge on  climatic  station  environmental  requirements  v/as  very 
meager.   However,  as  data  v/ere  collected  and  analyzed,  the  im- 
portance of  certain  environmental  effects  became  apparent. 
Stations  were  relocated  to  sites  where  they  were  surrounded  by 
an  extensive  area  of  vigorous,  low-growing  vegetation  at  full 
ground  cover.   Large,  well -managed  pastures  best  meet  these 
requirements.   At  such  sites,  the  confounding  effects  of  micro- 
environment  differences  are  minimized. 

The  techniques  used  for  the  determination  of  evapo- 
transpiration were  the  best  available  methods  for  the  task,  at 
the  time  they  were  employed   Hov/ever,  as  the  study  progressed, 
techniques  were  modified  to  take  advantage  of  new  and  better 
tools  as  they  became  available.   The  initial  soil  moisture 
measureme.its  to  determine  evapotranspiration  xvere  made  by  the 


-72- 


gravimetric  technique.   The  development  and  refinement  of  the 
neutron  scattering  technique  offered  promise  of  a  far  superior 
method  of  making  soil  moisture  determinations.   For  this  reason, 
this  new  equipment  was  adopted  shortly  after  it  became  commercially 
available. 

Small  evapotranspirometer  tanks  of  various  designs 
were  installed  and  used  where  high-v;ater  table  conditions  pro- 
hibited the  use  of  soil  moisture  depletion  techniques,  and  were 
later  installed  on  sites  where  no  high  water  tables  existed.   The 
success  of  these  devices  has  encouraged  the  extension  of  this 
method  to  other  close  growing  crops. 

Estimates  of  evapotranspiration  were  made  for  all 
areas  studied,  using  pan  and  atmometer  coefficients  and  evapora- 
tion data  collected  as  part  of  the  agroclimatic  program.   These 
estimates  were  compared  to  Bulletin  2  consumptive  use  values, 
using  the  Bulletin  2  growing  seasons.   In  many  cases,  the  esti- 
mates obtained  by  using  the  evaporation  correlation  technique 
were  higher  than  were  the  Bulletin  2  values. 

Data  collected  at  the  evapotranspiration  field  plots 
indicate  that  the  actual  periods  of  active  growth  are  considerably 
longer  than  those  assumed  in  the  determination  of  Bulletin  2 
values.   On  a  yearly  basis,  the  estimates  sho\m  in  this  report 
may  show  even  a  greater  variance  with  Bulletin  2  values. 

As  the  estimated  values  presented  in  this  report  are 
based  upon  only  two  years  of  record,  they  should  be  used  with 
considerable  caution.   However,  the  evaporation  correlation 


-73- 


technique  appears  to  promise  a  reasonable  means  of  estimatlnf 
with  precision  heretofore  unknovm,  evapotransplratlon  rates  I 
crops  In  the  various  geographic  areas  of  California. 

Conclusions 

1.  Correlation  of  evaporation  with  evapotransplratlf 
appears  to  promise  a  reasonable  means  of  estimating  evapotranS' 
plratlon  within  the  various  agricultural  area  of  the  State. 

2.  Reasonable  estimates  may  be  obtained  by  using 
either  pan, or  atmometer  coefficients. 

3.  Pan  and  atmometer  coefficients  are  strongly  in- 
fluenced by  percent  of  ground  cover,  particularly  for  ground 
cover  percentage  less  than  (60^o)  sixty  percent. 

^1-.   Estimated  values  presented  In  this  report  are 
based  upon  only  two  years  of  record,  and  so  should  be  used 
with  considerable  judgment. 

5.  On  the  basis  of  the  agroclimatic  data  collected, 
no  definite  segregation  of  the  State  into  areas  of  uniform 
evaporation  is  possible  at  present.  Inland  areas  appear  to  ha 
more  uniform  evaporation  rates  than  expected,  although  effect 
of  microenvlronment  cause  large  differences  of  evaporation  be- 
tween individual  measurement  sites. 

6.  It  may  be  found  that  the  len,^;th  of  growing  seaso: 
is  the  most  Important  factor  affecting  seasonal  evapotransplra 
tlon  in  inland  areas. 


■  74- 


Recommendations 
On  the  basis  of  the  collection  and  analysis  of  the 
data  on  vegetative  v;ater  use^  as  presented  in  this  report^  and 
on  the  conclusions  drav;n  therefrom,  it  is  recommended  that: 

1.  The  evapotranspiration  studies  at  the  present 
sites  be  continued  until  sufficient  data  can  be  collected  to 
provide  reasonable  estimates  of  evapotranspiration  under  the 
range  of  climatic  conditions  vjhich  can  occur  at  these  locations. 

2.  Additional  sites  for  evapotranspiration  measure- 
ments be  established  in  locations  having  different  climatic 
conditions  than  those  now  being  measured  to  determine  variability 
of  evapotranspiration  coefficients  (  i.e..  Delta  area,  coastal 
area^  and  desert  areas). 

3.  The  scope  of  the  present  program  be  expanded  to 
include  measurements  of  applied  water  under  different  irrigation 
practices  and  lengths  of  grov.'ing  seasons  for  major  crops  within 
the  various  agricultural  zones  of  the  State.   This  would  provide 
the  basic  information  needed  to  determine  irrigation  efficiencies, 
drainage  requirements,  and,  with  the  unit  evapotranspiration 
values,  to  determine  total  irrigation  water  requirements. 


-75- 


APPENDIX  A 


Supplemental  Agroclimatic   and   iilvapotranspiration  Data 


-77- 


TABLE  OP  CONTENTS 


Number  Pap;e 

A-1         Agrocllmatic  Stations,  Location  and  General 

Information 79 

A-2         Monthly  Evaporation  From  Standard  U,  S.  V.'eather 

Bureau  Evaporation  Pans „  , 85 

A-3  Monthly  Evaporation  Differences  Between 

Livingston  Spherical  Black  and  V/hlte  Atmometers  .   90 

A-4         Location  of  Evapotransplratlon  Measuring 

Stations , 98 

A-5         General  Information  Relative  to  Evapotransplratlon 

Measuring  Stations   ,  .  ,  „ IOC 

A-6         Neutron  Probe  Measurements  of  Evapotransplratlon 
and  Related  Data  for  Several  Irrigated  Crops, 
1959  and  i960 107 

A-7         Evapotransplrometer  Measurements  and  Related  Data 
for  Hlgh-VJater  Table  Pasture  and  Irrigated 
Ryegrass II3 


-78- 


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iGROCLIMATIC    STATIONS 


ACTlvE-1960 


Macdoel  F.  S. 

39. 

Bella  Vista  4NE 

76. 

Arbuckle   IS 

Montague  3NE 

40. 

Eagle  Lake  Stone  Ranch 

77. 

Lincoln  Vineyard 

Yreka  INE 

41. 

Hayfork  R.S. 

78. 

Auburn  Mt.   Vernon 

Davis  Creek  4WNW 

42. 

Redding  R.  S. 

79. 

Gold  Hill  Doty  Flat 

Grenada  6E 

43. 

Redding  Stayer 

80. 

Rocklin   Igarashi 

Fort  Jones  R.  S. 

44. 

Redding  6SE 

81. 

Woodfords 

Gazelle   INNE 

45. 

Redding  A.   P. 

82. 

Davis  Campbell  #1 

Gazelle  3NNW 

46. 

Anderson  2E 

83. 

Davis  Campbell  #2 

Big  Sage  Reservoir 

47. 

Anderson  3E 

84. 

Coleville  2W 

CedarviUe  Chevron 

48. 

Anderson  4E 

85. 

Elk  Grove  4NW 

CedarviUe  2E 

49. 

Leavitt  Lake 

86. 

Bridgeport  DWR 

CedarviUe   IE 

50. 

Standish  4NW 

87. 

Thornton  25 

Alturas  Park  Avenue 

51. 

Standish  INW 

88. 

Twitchell  Island 

Alturas  Dorris  Ranch 

52. 

Red  Bluff  3E 

89. 

Lodi  3SW 

Canby  Ohm 

53. 

Red  Bluff  Cone  Ranch 

90. 

Lodi  3S 

Canby  R.  S. 

54. 

Corning  3NW 

91. 

Stockton  8S 

Canby  1 ISW 

55. 

Cormng  3NE 

92. 

Stockton  9S 

Callahan  Towne  Ranch 

56. 

Corning  Jobe 

93. 

El  Solyo  Ranch 

Mt.  Shasta  City  W.B. 

57. 

Vina  Beck 

94. 

Vernalis  3SE 

Likely  Williams  Ranch 

58. 

Quincy  R.  S. 

95. 

Ceres    3E 

Likely  4N 

59. 

Newville 

96. 

Atwater   IN 

Adin  Harper 

60. 

Loyalton  5W 

97. 

Newman   ISE 

Adin  R.S. 

61. 

Loyalton  7N 

98. 

Merced  5SE 

West  Valley  Reservoir 

62. 

Hamilton  City 

99. 

Berenda  2N 

Lookout    IS 

63. 

Mills  Orchard 

100. 

Los  Banos  33 

Lookout  Hunt 

64. 

Oroville  Agric.  Comm. 

101. 

Los  Banos  Equipment  Yard 

Bieber  4E 

65. 

Richvale   IE 

102. 

Los  Banos  8SE 

Bieber  S.C.S. 

66. 

Sacramento  Refuge 

103. 

Kerman  2ESE 

McArthur  2E 

67. 

Palermo  3SW 

104. 

Fresno  Kearney  Pa.  k 

PittviUe   IS 

68. 

Pennington   3NW 

105. 

Mendota  Murietta  Ranch 

Glenburn  DWR 

69. 

Live  Oak  3SE 

106. 

Panoche  Junction 

Fall  River  Mills  4NW 

70. 

Loma  Rica 

107, 

Kingsburg  5S  #1 

Fall  River  Mills  R.S. 

71. 

Browns  Valley  3NE 

108. 

Kingsburg  58  #2 

Fall  River  Mills  Intake 

72. 

Penn  Valley 

109. 

Shatter  2NW 

Madeline   3SW 

73. 

Tahoe 

110. 

Arvin  Frick 

Termo 

74. 

Yuba  City 

111. 

Arvin  Jewett  #1 

Hat  Creek  3N 

75. 

Yuba  City  9W 

112. 

Arvin  Jewett  #2 

Hat  Creek  3SE 

STATE  OF  CALIFORNIA 

THE   RESOURCES  AGENCY   OF  CALIFORNIA 

DEPARTMENT  OF  WATER  RESOURCES 

DIVISION   OF   RESOURCES   PLANNING 

VEGETATIVE  WATER    USE    STUDIES 

INTERIM  REPORT 

GENERAL    LOCATION 


AGROCLIMATIC    STATIONS 
1954-1960 


0     R     E     G     O 


Montague  3NE 

Yreka  INE 

Davis  Creek  4WNW 

Fort  Jones  R,  S. 
Gazelle   INNE 


Big  Sage  Reservoi 
Cedarville  Chevrc 
Cedarville  2E 


Canby   1 ISW 
Mt.  Shasta 


Likely  4N 
Adin  Harper 
Adin  R.S. 
West  Valley  Res. 
Lookout   IS 
Lookout  Hunt 


Mills  4NW 
Mills  R.S. 
Mills  Intake 


.  Vis 


I  4NE 


Eagle  Lake  Ston. 
Hayfork  R.S. 
Redding  R.  S. 
Redding  Stayer 

Redding  A.  P. 
Anderson  2E 
Anderson  3E 
Anderson  4E 


Mills  Ore 
Oroville  . 

Penningto 


Gold  Hill  Doty  Flat 

Woodfords 

Davis  Campbell  #1 


Berenda  2N 

Los  Banos   3S 

Los  Banos  Equipment  Yar, 


05.     Mendota  Mur: 


THE  RESOURCES  AGENCY  OF  CALIFORNIA 

DEPARTMENT   OF   WATER    RESOURCES 

DIVISION  OF  RESOURCES   PLANNING 

VEGETATIVE  WATER    USE    STUDIES 

INTERIM  REPORT 

GENERAL    LOCATION 

AGROCLIMATIC    STATIONS 
1954-1960 


THE   RESOURCES   AGENCY  OF  CALIFORNIA 

DEPARTMENT   OF   WATER    RESOURCES 

DIVISION  OF  RESOURCES   PLANNING 


GENERAL    LOCATION 


EVAPOTRANSPIRATION    STATIONS 
1955-1960 


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CONDITIONS,  SOIL    MOISTURE    avAILdBILITY 
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CODE:    •    EVAPOTRANSPIRATION  MEASURED 
o    EVAPOTRANSPIRATION  ESTIMATED 


THE   RESOURCES   AGENCY  OF  CALIFORNIA 
DEPARTMENT   OF   WATER    RESOURCES 

DIVISION   OF   RESOURCES   PLANNING 

VEGETATIVE  WATER   USE    STUDIES 

INTERIM   REPORT 

COMPARISON  OF  EVAPOTRANSPIRATION  CURVES 

OF 

DIFFERENT    CROPS    GROWN 

AT    THE 

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VARIATION  OF  PAN  AND  ATMOMETER  COEFFICIENTS 

FOR 

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WITH   RESPECT    TO 

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TMOMETER    COEFFICIENTS 


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90  100 


PERCENT  GROUND   COVER 


THE  RESOURCES  AGENCY  OF  CALIFORNIA 

DEPARTMENT   OF  WATER    RESOURCES 

DIVISION  OF  RESOURCES  PLANNING 

VEGETATIVE    WATER   USE    STUDIES 
INTERIM    REPORT 

RELATIONSHIP  BETWEEN 
PAN  AND   ATMOMETER  COEFFICIENTS 
FOR   ALFALFA  AND   GROUND   COVER 

DATA    ARE    FOR    1959  AND  I960 


evAPOTRANSPIRATION   STATIONS 
EVAPOTRANSPEROMETER -ACTIVE   IN  I960 
EVAPOTRANSPEROMETER- ACTIVE  PRE  I960 
NEOTRON    PROBE -ACTIVE    IN    i960 
NEUTRON    PROBE -ACTIVE    PRE    I960 
GRAVIMETRIC- ACTIVE   PRE    i960 


Gazelle  Dougherty  #1 
Gazelle  Dougherty  #2 
Gazelle  Dougherty  #3 
Canby  Bus  hey 
Alturas  Dorris  Ranch 
Bieber  3E 
Bieber  Leonard 
PittviUe  (AA) 
McArthur  (AB) 
McArthur    INE 
McArthur  Albaugh  #1 
McArthur  Albaugh  #2 
Pittville   IS 
Hat  Creek  Kern 
Hat  Creek  Opdyke 
Redding  6SE 
Anderson  2N 
Anderson   3E 
Anderson  Trisdale 
Leavitt  Lake 
Mills  Orchard 
ColeviUe  2W 
Davis  Cannpbell 
Arvin  (CE) 
Arvin  (CO 
Arvin  (CB) 
Arvin  (CF) 
Arvin  (CD) 


in  Je 


#2 


STATE  OF  CALIFORNIA 

THE   RESOURCES   AGENCY   OF  CALIFORNIA 

DEPARTMENT  OF  WATER  RESOURCES 

DIVISION   OF   RESOURCES   PLANNING 

VEGETATIVE  WATER    USE    STUDIES 

INTERIM    REPORT 

GENERAL    LOCATION 
EVAPOTRANSPIRATION    STATIONS 

1955-1960 


/A. 


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